Preheat the oven to 350F. Grease two 9" cake pans w/ butter & dust w/ flour. Line the bottoms w/ parchment paper rounds.

Cake Layers: In a bowl, whisk together the flour, sugar, cocoa powder, baking soda/powder & salt.

In a separate large bowl, whisk together the buttermilk, oil, eggs & vanilla.

Add dry ingredients to wet; whisk until combined. Pour in the hot coffee & stir quickly (batter will be thin).

Divide batter evenly among the cake pans. Bake at 350F for 30-35 mins, until a toothpick comes out clean.

Set on a wire rack to cool completely in the pan.

As they cool, make chocolate mousse. In a bowl, beat the eggs w/ the sugar for ~2-3 mins, until pale & foamy.

Slowly beat in ½ of the heavy cream. Pour mixture into a medium saucepan & place over medium heat. Stir constantly while it thickens, ~3-4 mins (DON'T boil). Remove.

Add choc chips & let sit for 1 min, then mix. Pour mixture into a bowl & chill in the fridge for 1hr.

1hr before assembling, beat the remaining heavy cream in a large bowl until soft peaks form.

Add spoonfuls of the chocolate mixture as you continue beating, until smooth & fluffy. Place in the fridge.

Assemble: I assemble this in a 9" springform pan w/ wax paper around the edges (you can do it on a cake stand). Level the cakes by slicing a thin layer off the tops. Place 1 cake layer down. Spoon ~⅓ cup of the ganache (save rest for topping) on top & smooth. Add all of the chocolate mousse on top & smooth. Top w/ the 2nd cake layer. Chill for at least 1hr.

Serve: Pour the rest of the ganache over the chilled cake. Garnish w/ flaky sea salt.