Preheat the oven to 170°C (340°F).
Combine the dry ingredients - flour, baking powder, baking soda, salt, cocoa powder, and coffee powder—in a bowl. Give them a quick whisk.
Next, add in the brown sugar, white sugar, buttermilk, oil, egg, and vanilla extract. Mix well until you have a smooth batter with no lumps.
Pour in the hot water gradually, whisking as you go until the mixture is fully incorporated.
Prepare two 6-inch cake pans by greasing them with butter and dusting lightly with flour. Divide the batter equally between the pans and bake for 15 to 20 minutes. The cakes are done when a toothpick inserted in the center comes out clean.
Allow the cakes to cool completely in the pans. Once cooled, wrap them tightly in cling film and refrigerate until you're ready to assemble.