In a large mixing bowl, add softened butter and sugar. Mix them on low speed until combined. Increase speed to medium and mix for about at least 10 minutes. This will give a pale, creamy and fluffy batter.

Add in the eggs, one at a time.

Add in the dry ingredients, spoon by spoon.

Add vanilla extract and milk. Continue mixing on low speed for about 1 to 2 minutes or until all ingredients are fully incorporated.

Prep the chocolate flavor by mixing the cocoa powder and hot water. I'm excluding the instant coffee powder.

Dive the cake batter into two. Add in the chocolate mixing into one of the halves. Mix the chocolate until well incorporated.

Add a few spoons of white batter into the prepared pan, covering the bottom of the pan. Then add in 2 table spoons the chocolate batter in the middle. Gently stir in the middle a little bit so that the batter will slowly spread to the sides. Then do the same for the white batter again, and alternatively chocolate and then white batter.

Bake at preheated oven 160 C/ 320 F for 50 to 60 minutes, depending on individual oven and cake.