Perfect Chocolate Cake

Start cake: Place cocoa in a medium bowl. Gently pour boiling water over top and stir to combine; let cool, about 15 minutes.

Meanwhile, preheat the oven to 350 degrees F (175 degrees C). Grease and flour three 9-inch round cake pans.

Sift together flour, baking soda, salt, and baking powder into a mixing bowl.

Combine sugar and butter in a large mixing bowl; beat with an electric mixer until creamy. Add eggs, one at a time, mixing well after each addition. Stir in vanilla. Add flour mixture alternately with the cooled cocoa mixture, mixing only until combined. Pour 1/3 of the cake batter into each of the prepared pans, spreading evenly.

Bake in the preheated oven until a toothpick inserted into the centers comes out clean and cakes pull away from the sides of the pans, 18 to 22 minutes. Remove cakes from the oven and set on a wire rack to cool for about 5 minutes. Run a table knife around the edges to loosen. Invert cakes carefully onto the wire rack and let cool completely, about 20 more minutes.

While the cakes are cooling, make frosting and filling: Beat butter for frosting in a mixing bowl until creamy. Gradually mix in confectioners' sugar, cocoa, and vanilla; continue to mix until light and fluffy, 7 to 10 minutes. Set aside.

Make filling: Whip heavy cream and vanilla in a bowl until cream becomes thick. Add confectioners' sugar and continue to whip until stiff but not too grainy.

Fill and frost cake: Remove cooled cake layers from pans. Trim any top surfaces to make them flat. Place one layer on a nice plate and spread 1/3 of the filling over the top. Top with another layer and another 1/2 of the remaining filling. Place the remaining cake layer on top. Spread frosting around the sides of the layers, leaving a ridge that extends up over the top edge. Spread the remaining filling over the top of the cake, inside the ridge of frosting.