Happy Pi(e) Day!

My daughter made whoopie pi(es) to celebrate Pi Day (3/14) with the Pierson Middle School Math Club. Here's the recipe:

Whoopie Pi(es)
Makes 20 small pies

For the cakes:
1 cup unsweetened natural cocoa powder
1 cup boiliong water
1/2 cup sour cream (not low- or non-fat)
1 large egg yolk
1 teaspoon vanilla extract
2 cups unbleached all-purpose flour
1 cup firmly packed light brown sugar
1 teaspoon baking soda
1/2 teaspoon salt
1/2 cup (1 stick) unsalted butter

For the filling:
5 tablespoons unsalted butter, softened
1/2 cup confectioners' sugar
1 tablespoon light corn syrup
1/2 teaspoon vanilla extract
Pinch salt
3/4 cup Marshmallow Fluff

1. Make cookies: Preheat oven to 375 degrees. Line baking sheets with parchmet paper.
2. Whisk together cocoa powder and boiling water in a small bowl until smooth. Set aside to cool. Combine sour cream, egg yolk, and vanilla in a medium mixing bowl.
3. Combine flour, brown sugar, baking soda, and salt in a large mixing bowl. Add butter an mix with an electric mixer on low speed until butter pieces are no bigger than small peas. Stur in sour cream mixture until ingredients are moistened. Stir in cocoa mixture and beat well to combine.
4. Use a small ice cream scoop to drop batter by tablespoonfuls onto prepared baking sheets, leaving an inch between each cookies. Bake until tops are cracked, 8 to 9 minutes. Remove sheets from oven and let cookies stand on sheets for 5 minutes to firm up. Transfer to a wire rack to cool completely.
5. Make filling: Mix together butter and confectioners sugar in a mixing bowl with an electric mixer until combined. Stir in corn syrup, vanilla, and salt. Stir in Fluff and beat on high until light and fluffy, about 3 minutes.
6. Spoon 1 teaspoon of filling onto the flat side of a cake. Sandwich with another cake. Repeat with remaining cookies.