Find a square baking pan 30x20 centimeters and line it with baking paper, then set the oven to 200 degrees Celsius (392 degrees Fahrenheit).
Take a bowl and mix all the dry ingredients in it.
Add oil and buttermilk.
Stir the bowl with a regular whisk – do not use an electric mixer.
Stir until the batter is smooth and lump-free.
Take your baking pan and pour the batter in, making sure to smooth the surface with a spoon or spatula.
Bake the cake for about 30 minutes.
Cool the cake after baking.
Coffee buttercream
Mix the cooled coffee with butter, salt, and powdered sugar; whisk until the consistency is a thick buttercream.
Serving
Take your coffee buttercream and spread a good layer over the entire cake.
Then sprinkle hazelnut flakes over the cake, with a sprinkle of flaky salt.