Step 1: Prepare the Cake Batter

1. Preheat the oven to 350°F (175°C). Grease and line two 8-inch cake pans with parchment paper.
2. In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Whisk well and set aside.
3. In another bowl, whisk together the granulated sugar, brown sugar, and vegetable oil until smooth. Add the eggs one at a time, whisking after each addition.
4. Stir in the vanilla extract and buttermilk, mixing until well combined.
5. Gradually add the dry ingredients to the wet ingredients, alternating with the cooled brewed coffee. Mix until the batter is smooth and fully incorporated.

Step 2: Bake the Cake

1. Divide the batter evenly between the prepared cake pans.
2. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
3. Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Step 3: Make the Mocha Buttercream Frosting

1. In a large mixing bowl, beat the butter until light and fluffy.
2. Gradually add the powdered sugar and cocoa powder, beating on low speed until combined.
3. Pour in the coffee, vanilla extract, and salt, then mix well.
4. Add heavy cream or milk, one tablespoon at a time, until the frosting reaches a smooth and spreadable consistency.

Step 4: Assemble the Cake

1. Place one cake layer on a serving plate. Spread a layer of frosting evenly over the top.
2. Add the second cake layer and frost the top and sides of the cake with the remaining buttercream.
3. Garnish with chocolate shavings, coffee beans, or a dusting of cocoa powder for an elegant finish.

Step 5: Serve and Enjoy

Slice and serve this decadent mocha cake with a cup of coffee or tea. Enjoy the perfect balance of chocolate and coffee flavors in every bite!