Mini Chocolate Cakes with Dark-Chocolate Ganache

Cakes: Preheat oven to 375 degrees. Brush mini muffin tins with butter, then dust with cocoa powder, tapping out excess. Melt chocolate, stirring until smooth, in a bowl set over a saucepan of simmering water; remove from heat and let cool. Stir together cocoa powder and hot water in a small bowl until a smooth, loose paste forms; set aside.

Whisk together flour, baking soda, and salt in a medium bowl. Beat together butter and sugars with a mixer on medium-high speed until light and fluffy. Reduce speed to low and beat in eggs, 1 at a time, until incorporated. Beat in vanilla, cocoa mixture, and melted chocolate. Beat in half of flour mixture, then sour cream. Beat in remaining flour mixture just until combined, scraping down sides of bowl as needed.

Spoon 2 tablespoons batter into each mini muffin cup. Bake until puffed and a toothpick inserted into center of cakes comes out with moist crumbs attached, 8 to 10 minutes. Let cakes cool in tins 5 minutes, then invert onto a wire rack to let cool completely.

Ganache: Bring cream to a simmer in a small saucepan over high heat. Place chocolate in a small bowl. Pour cream over chocolate, let stand 1 minute, then stir until chocolate is melted and smooth.

Assembly: Pour 1 teaspoon ganache over each cake. Let stand until ganache is set, about 30 minutes. Shave block chocolate with a peeler and place curls on top of cakes.