Preheat your oven to 350 degrees F and grease and line an 8-inch round cake pan with parchment paper.

In a bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In a separate large bowl, whisk the granulated sugar, brown sugar, eggs, buttermilk, oil, and vanilla until smooth.

Slowly pour in the hot coffee and mix gently.

Stir the dry ingredients into the wet just until combined.

Pour the batter into the prepared pan and bake for 28 to 35 minutes.

Let the cake cool completely in the pan.

Whip the cold heavy cream and powdered sugar until soft peaks form.

Gently fold in the melted chocolate in batches.

Spread the chocolate mousse evenly over the cooled cake layer and refrigerate for at least 2 hours.

Heat the heavy cream just until steaming.

Pour it over the chopped chocolate and let sit for 2 to 3 minutes.

Stir until smooth and glossy.

Pour the ganache over the chilled mousse layer and refrigerate another 30 to 60 minutes until set.

Let the cake sit at room temperature for about 10 minutes before slicing.

Lift each slice with a wide, thin knife warmed briefly under hot water and wiped dry.

Serve on a simple white plate with optional flaky sea salt or toasted hazelnuts.

A small scoop of vanilla bean ice cream can enhance each bite.