Best-Ever Chocolate Cupcakes

Preheat oven to 350°. In a liquid measuring cup or a small bowl, stir together hot water and espresso powder; let cool. Meanwhile, in a large bowl, sift together flour, cocoa powder, baking soda, baking powder, and salt. In the bowl of a stand mixer fitted with the whisk attachment (or a large bowl using a handheld electric mixer), add eggs, milk, oil, and vanilla and whisk on medium speed until just combined. Add sugar and espresso mixture and mix until incorporated. Add flour mixture and mix on low speed, scraping down sides of the bowl as needed, until fully incorporated and mixture looks glossy, 2 minutes. (The mixture will be runny.) Line two cupcake pans with enough liners for 20 cupcakes. Fill each two-thirds of the way with batter (for ease, use a 3-tablespoon cookie or ice cream scoop). Bake, rotating pans after 15 minutes, until a cake tester or wooden skewer inserted into the center of cupcakes comes out clean, about 20 minutes. Remove and transfer to a baking rack. Let cool completely. To serve, pipe each cupcake with a generous amount of marshmallow buttercream. Top with mini marshmallows and chocolate sprinkles or finely grated chocolate.