Boston Cream Whoopie Pies

Happy Pi Day! I don't know if I've ever told you guys what a huge math nerd I was (am?) but it's true. Before I went to law school, I graduated with a degree in engineering, so math and science are sort of my thing. Shane's too, for that matter - he's also an engineer! So it only makes sense that I'd be excited to celebrate Pi Day, especially since I actually remembered a day in advance this year. Usually it's not until I start seeing everyone's posts pop up in my Google Reader that I remember it's March 14, but I'm a little more on the ball this year.



The only problem? As much as I like eating pie (blueberry and apple are my favorite), I'm still not a huge fan of making it. I thought about what I could make instead of a traditional pie that would still be appropriate, and I kept coming back to whoopie pies. With the word pie in the name, I decided that technically they still fall within the rules. Right? Ok, good :)



I've had a recipe for Boston cream pie bookmarked for a while, but haven't had an occasion to make it, so this seemed like the perfect opportunity to use a few components of that recipe to make these Boston cream whoopie pies. Vanilla cookies filled with a rich pastry cream and topped with a smooth chocolate glaze - a perfect handheld version of Boston cream pie! These do require a bit of advanced planning, as the pastry cream needs at least a few hours to set up in the fridge before assembly. I've failed at pastry cream more times than I care to tell you, but this recipe yielded success for me finally, yay! Before sending these whoopie pies off to work with Shane, I topped them with a few sprinkles in the name of celebration - happy Pi Day everyone :)

Boston Cream Whoopie Pies
cookies from Annie's Eats (who adapted from King Arthur Flour), pastry cream and glaze from Cook's Illustrated

Pastry Cream
1 cup half-and-half
3 large egg yolks
1/4 cup sugar
pinch salt
2 tablespoons all-purpose flour
2 tablespoons cold unsalted butter, cut into two pieces
3/4 teaspoon vanilla extract

Cookies
2 1/4 cups all-purpose flour
1 1/2 teaspoons baking powder
3/4 teaspoons salt
8 tablespoons unsalted butter, at room temperature
3/4 cup sugar
1 large egg plus 1 large egg white, at room temperature
1 teaspoon vanilla extract
1/2 cup milk (I used whole)

Glaze
1/4 cup heavy cream
2 oz bittersweet or semisweet chocolate, finely chopped
1 teaspoon light corn syrup (optional)

To make the pastry cream: Add the half and half to a medium saucepan and set over medium heat. Cook until the the half and half comes to a simmer. Meanwhile, in a medium bowl, whisk the egg yolks, sugar, and salt until smooth, then add the flour and whisk to incorporate completely. When the half and half reaches a simmer, remove the pan from the heat, and, whisking constantly, slowly drizzle it into the yolk mixture. Add the now warmed yolk mixture back to the saucepan.

Set the pan over medium heat and cook, whisking constantly, until the mixture thickens slightly. Reduce the heat to medium-low and continue cooking, still whisking constantly, until bubbles burst on the surface of the mixture (this took about 4-5 minutes total for me). Turn off the heat under the pan, and whisk in the butter and vanilla until completely incorporated. Strain the pastry cream through a fine-mesh strainer into a heatproof bowl. Spray a piece of parchment paper with nonstick cooking spray, then press the parchment against the surface of the pastry cream. Refrigerate for at least 2 hours, or up to 24, before using.

To make the cookies: Preheat oven to 350 F. Line two baking sheets with parchment paper.

Whisk the flour, baking powder and salt together in a medium bowl. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium speed until light and fluffy, about 3 minutes. Add the egg and egg white and beat until well combined, stopping to scrape down the sides of the bowl once. Mix in the vanilla. With the mixer on low, alternately add the dry ingredients in 2 additions, and the milk in 1, starting and finishing with the dry ingredients. Beat just until incorporated (the batter will be fairly thick).

Transfer the batter to a pastry bag fitted with a large plain round tip. Pipe the batter into about 1 1/2-inch rounds onto the prepared baking sheets, leaving about 1-2 inches between the cookies. (Note: I tried to make a few cookies using a cookie scoop to portion the batter instead of piping, and they came out really jagged so while I won't say don't do it, I advise against it.) Bake for 8-9 minutes, or until the cookies spring back when lightly pressed (they won't take on much color). Transfer the baking sheet to a wire rack and let the cookies cool for 5-10 minutes before removing them to the rack to cool completely.

To make the glaze: Add the cream to a small saucepan. Bring to a simmer, then add the chocolate and corn syrup (if using - the syrup just makes the glaze a bit more shiny, but you don't need it.) Let stand for about 1 minute, then gently whisk the mixture until the chocolate is completely melted and smooth. Set the mixture aside to thicken slightly, until the consistency is right for spreading - about 5-10 minutes.

To assemble: Match the cookies in pairs by size. Spread the top (the rounded side) of one cookie in each pair with a little bit of the glaze. Let the glaze set (you can transfer the cookies to the fridge to speed this up). Transfer the pastry cream to a pastry bag fitted with a small round tip. Pipe about 1 teaspoon of pastry cream onto the flat side of the other cookie in each pair. Top with the glazed cookie and press lightly to push the pastry cream to the edges.

Makes about 24 sandwich cookies (you will probably have a little extra pastry cream)