Preheat your oven to 350°F (175°C). Grease a 9-inch round baking pan and line the bottom with parchment paper.

In a large bowl, sift together flour, cocoa powder, granulated sugar, brown sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together buttermilk, oil, egg, and vanilla extract.

Gradually add the wet ingredients into the dry ingredients, stirring until just combined.

Slowly mix in the hot water or coffee until the batter is smooth. Be careful not to overmix.

Pour the batter into the prepared baking pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 15-20 minutes, then transfer it to a wire rack to cool completely. Wrap in plastic wrap and chill in the refrigerator for at least 1 hour for easier frosting.

In a stand mixer or large bowl, beat the butter on medium speed for 2 minutes until light and fluffy.

Add vanilla extract and continue mixing.

Gradually add powdered sugar, ½ cup at a time, mixing on low speed to avoid a sugar cloud.

Pour in heavy cream and a pinch of salt, then increase speed to medium-high and beat for 4-5 minutes until smooth and fluffy.

Place a small dollop of frosting in the center of a cake stand or serving plate to hold the cake in place.

Position the cake on top and spread a generous amount of buttercream over the top using an offset spatula.

Smooth out the frosting and decorate with sprinkles, chocolate shavings, or fresh berries.

Slice and enjoy!