Gently heat 1 cup of heavy cream and 1/2 cup of dark brown sugar for 3–4 minutes until dissolved. Pour over 8 ounces of chopped dark chocolate and whisk until smooth. Refrigerate until thickened, about 45 minutes.

In a large bowl, whisk together 1 3/4 cups flour, 1 cup dark brown sugar, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. In another bowl, mix 2 eggs, 1 teaspoon vanilla, 1 cup hot water, and 2 tablespoons instant coffee, then mix into dry ingredients until smooth.

Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. Pour batter into pans and bake for 30–35 minutes until a toothpick comes out clean. Cool in pans for 10 minutes before transferring to wire racks.

Mix 1 cup hot water and 1 tablespoon instant coffee until dissolved. Brush generously over cooled cake layers.

Carefully slice each layer horizontally to create four thinner layers. Layer with frosting between each layer, finishing with frosting on top.

Apply a thin crumb coat to seal in crumbs and chill for about 30 minutes. Remove and apply final layer of frosting.

Slice the cake and enjoy it chilled or at room temperature, optionally with whipped cream or vanilla ice cream.