4 tablespoons unsalted butter, at room temperature
1/2 cup light brown sugar, packed
1 large egg yolk
1/2 teaspoon vanilla extract
1 cup all-purpose flour
3 tablespoons unsweetened cocoa powder
1/4 teaspoon salt
1/2 teaspoon baking soda
1/2 teaspoon baking powder
1/2 teaspoon apple cider vinegar
1/2 cup buttermilk
1 recipe California Strawberry Whoopie Pie Filling
3 medium California strawberries, halved, for garnish
Preheat oven to 350°F. Lightly spray whoopie pie pan with non-stick cooking spray and set aside.
Place butter and sugar in a large bowl and beat with an electric mixer until fluffy. Add egg yolk and vanilla, beating to combine.
In another large bowl, whisk together flour, cocoa powder, salt, baking soda and baking powder.
Gradually add flour mixture to butter mixture, beating constantly.
Pour in vinegar and buttermilk and beat at low speed just until combined.
Spoon batter into prepared pan. Bake until center of cake is slightly puffed, 9-10 minutes.
Remove from heat and let cakes cool in pan for 10 minutes. Remove from pan and let cool on a wire rack to room temperature before filling.
To assemble, spread filling very thickly to cover the bottom of 6 pies. Place remaining 6 pies on top of filling. Top each pie with a strawberry. Serve immediately.
California Strawberry Whoopie Pie Filling
Makes filling for 6 pies. This filling can be made up to 1 week ahead. Cover filling and refrigerate.
1 cup diced California strawberries
2 tablespoons unsalted butter, at room temperature
2 tablespoons marshmallow crème
1 3/4 cup powdered sugar
Place strawberries and butter in a small saucepan over medium high heat, stirring constantly until mixture starts to bubble, about 3 minutes. Reduce heat to medium-low and simmer, stirring occasionally, until very thickened, about 15 minutes. Remove from heat and let cool completely.
Place butter and strawberry reduction in a large bowl and beat with an electric mixer until smooth and creamy. Beat in marshmallow crème. Add sugar, ¼ cup at a time, until mixture is stiff. Cover until ready to use.