Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour two round cake pans.
Mix Dry Ingredients: In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.
Combine Wet Ingredients: In a separate bowl, whisk together the eggs, egg whites, buttermilk, vegetable oil, lemon zest, lemon juice, and vanilla extract.
Add Coffee: Slowly pour in the hot brewed coffee and mix until the batter is smooth.
Combine Wet & Dry: Gradually add the wet ingredients into the dry ingredients, stirring until fully combined.
Bake the Cake: Divide the batter evenly between the two prepared pans and bake for 30-35 minutes, or until a toothpick comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
Prepare Lemon Frosting: Whip heavy cream in a mixing bowl until stiff peaks form. In a separate bowl, beat the egg whites and powdered sugar until fluffy. Gradually add the butter, lemon zest, and salt, and continue beating until smooth and fluffy.
Assemble the Cake: Place one layer of the cooled cake on a serving plate. Spread a layer of lemon frosting on top. Top with the second layer and frost the top and sides of the cake with the remaining frosting.
Chill and Serve: Refrigerate the cake for about 30 minutes before serving to set the frosting.