Preheat your oven to 350°F and prepare two 9” pans with parchment paper and butter.
In a big bowl, mix together flour, sugar, baking powder, baking soda, and salt.
Add in the eggs, oil, and buttermilk. Mix until just combined.
Mix instant coffee with hot water and add to the batter. Mix until combined.
Take 1/3 of the batter into another bowl, sift in the cocoa powder, and fold it in.
Pour the original batter into the two pans equally, then spoon on the cocoa batter. Swirl with a toothpick for a marbled effect.
Bake for 25-28 minutes until a toothpick comes out with moist crumbs. Let the cakes cool.
In a separate bowl, whisk together coffee, sugar, and water until it forms a light-colored, thick mixture (Dalgona).
In another bowl, whisk butter and 1.5 cups icing sugar until light and airy. Add the Dalgona mixture and whisk until smooth. Adjust to taste.
Once the cakes have cooled, trim off the tops, divide into two layers each, and assemble on a cake stand with buttercream between layers. Cover the top and edges with buttercream.
Refrigerate for 1 hour, then serve and enjoy your delicious coffee cake!