1. Preheat the oven to 350°F (180°C). Grease two 8 inch round cake pans, line the bottoms with parchment paper, and lightly flour the sides.

2. In a large bowl whisk together the all purpose flour, cocoa powder, instant espresso powder, baking powder, baking soda, and fine sea salt until well combined and no cocoa lumps remain.

3. In a separate mixing bowl whisk the granulated sugar, light brown sugar, and eggs until the mixture looks slightly thickened and glossy.

4. Add the whole milk, sour cream or Greek yogurt, neutral oil, and vanilla extract to the egg mixture and whisk until smooth and fully combined.

5. Pour the wet ingredients into the dry ingredients and gently stir with a spatula until mostly combined with just a few streaks of flour remaining.

6. Carefully pour in the hot brewed espresso or strong coffee and whisk or stir until the batter is smooth and evenly mixed; the batter will be fairly thin.

7. Divide the batter evenly between the two prepared cake pans and smooth the tops with a spatula to level.

8. Bake the cakes for 28 to 34 minutes or until the centers spring back when lightly pressed and a toothpick inserted in the middle comes out with a few moist crumbs.

9. Allow the cakes to cool in the pans for 10 to 15 minutes, then run a knife around the edges, turn them out onto a wire rack, peel off the parchment, and let the layers cool completely.

10. To make the mocha buttercream, beat the softened unsalted butter in a large bowl with a hand mixer or stand mixer until light and creamy, about 2 to 3 minutes.

11. Add the powdered sugar, cocoa powder, and instant espresso powder to the butter, mixing on low speed until the dry ingredients are mostly incorporated.

12. Pour in 3 tablespoons of heavy cream or milk, the vanilla extract, and fine sea salt, then increase the mixer speed and beat until the frosting is fluffy and spreadable, adding an extra tablespoon of cream if needed.

13. Place one cooled cake layer on a serving plate or cake stand and spread a generous layer of mocha buttercream over the top, pushing the frosting all the way to the edges.

14. Set the second cake layer on top, pressing gently to level, then frost the top and sides of the cake with the remaining buttercream, smoothing or swirling as desired.

15. Garnish the frosted cake with dark chocolate shavings or curls if using, then let the cake rest for at least 20 to 30 minutes before slicing so the frosting can set and the flavors meld.