Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter and dust them with cocoa powder to prevent sticking.

In a large mixing bowl, sift together the flour, cocoa powder, baking powder, and salt. Whisk until thoroughly combined.

In another bowl, whisk together sugar, eggs, and vegetable oil. Beat on medium speed until pale and fluffy, about 3-4 minutes.

Gradually add the dry ingredients to the wet mixture, alternating with buttermilk, until fully combined. Do not overmix.

Pour the batter evenly into the prepared cake pans and bake for 30-35 minutes. Check for doneness with a toothpick.

Allow the cakes to cool in the pans for about 10 minutes, then transfer to wire racks to cool completely.

Place one cooled cake layer on a serving plate and spread chocolate filling on top, then add the second layer.

Frost the top and sides of the cake with your choice of chocolate frosting and refrigerate for at least 1 hour to set.