Preheat the oven to 350°F (175°C). Grease and line an 8-inch round cake pan with parchment paper.
In a large mixing bowl, whisk together the melted butter, oil, granulated sugar, and brown sugar until smooth.
Add the eggs and vanilla extract, whisking until fully combined.
Pour in the buttermilk and mix well.
Sift in the flour, cocoa powder, baking soda, baking powder, and salt, then mix until just combined.
Slowly pour in the hot coffee (or water) while stirring gently until the batter is smooth and glossy. The batter will be thin—this is normal!
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.