Preheat your oven to 350F (conventional) and prepare a 9" cake pan with parchment paper and butter/oil.
If making your own buttermilk, mix ½ cup milk with ½ tablespoon vinegar, stir, and set aside.
In a bowl, mix the flour, sugar, baking powder, baking soda, and salt. Sift the ingredients to ensure no lumps.
Add the egg, vegetable oil, and buttermilk to the dry ingredients. Gently whisk until everything comes together.
In a smaller bowl, mix boiling water with cocoa powder and instant coffee. Whisk until you get a shiny mix, blooming the cocoa powder.
Add the bloomed cocoa powder mix to the cake batter and gently fold until combined.
Pour the batter into the prepared pan and bake at 350F for about 20-25 minutes until a toothpick comes out with moist crumbs attached.
Wait for 10 minutes, then turn out the cake onto a wire rack.