Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper for easy removal.
In a large bowl, whisk together the all-purpose flour, baking powder, kosher salt, and 1/2 cup of cocoa powder. Set this bowl aside.
In another bowl, mix the buttermilk, Greek yogurt, canola oil, eggs, granulated sugar, and vanilla extract until well blended.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Be careful not to overmix.
Stir in the black coffee, which will enhance the chocolate flavor of your cake.
Divide the cake batter in half. To one half, gently fold in the remaining 1/2 cup cocoa powder. In your prepared pans, alternate spoonfuls of vanilla and chocolate batter to create a zebra pattern.
Bake in the preheated oven for 30-35 minutes or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring to a cooling rack.
In a chilled bowl, whip the heavy cream until soft peaks form. In another bowl, mix the remaining 1/2 cup cocoa powder, honey or powdered sugar, 2 teaspoons vanilla extract, and a pinch of sea salt. Gently fold this mixture into the whipped cream until fully combined.
Once the cake layers are completely cooled, place one layer on a serving plate. Spread a generous layer of chocolate mousse on top, followed by the second cake layer. Top with additional mousse to cover the entire cake.
Chill the assembled cake in the refrigerator for at least 2 hours before serving. This will help the mousse set beautifully.
