1 cup (2 sticks) plus 2 tablespoons unsalted butter, room temperature
1/2 cup unsweetened cocoa powder
1 cup all-purpose flour
1 cup granulated sugar
3/4 teaspoon baking soda
1/2 teaspoon baking powder
1/4 teaspoon salt
1 large egg plus 1 large egg yolk
1/2 cup buttermilk
1/2 teaspoon pure vanilla extract
12 Oreo cookies
3 tablespoons seedless raspberry jam (if you can't find seedless, just warm it on the stove and strain it through a fine mesh sieve)
3 cups marshmallow topping
1/3 cup confectioners' sugar
Preheat oven to 350 degrees. Line 12 standard muffin cups with paper liners. Melt 2 tablespoons butter; let cool. In a large bowl, whisk together cocoa powder, flour, granulated sugar, baking soda, baking powder, and salt. Lightly beat egg and egg yolk and add, along with melted butter, buttermilk, vanilla, and 1/2 cup hot water; whisk until smooth. Spoon 1 tablespoon batter into each muffin cup and top each with 1 tablespoon batter. Bake until cupcakes are puffed and set, 15 minutes, rotating pan halfway through. Let cupcakes cool in pan on a wire rack.
Meanwhile, in a large bowl, using an electric mixer, beat 1 cup butter and raspberry jam on medium-high until smooth. Add marshmallow topping and confectioners' sugar and beat until fluffy and smooth, 3 minutes. Transfer mixture to a quart-sized ziploc bag; with scissors, snip a small hole in one corner/ Pipe frosting onto cooled cupcakes and serve immediately.