Turn on the oven to 175 deg C.
Combine butter and sugar in a mixing bowl. Whisk them together.
Add salt. Add one egg and whisk it. Then add another egg, then whisk further. That way, slowly add the eggs one by one and continue whisking.
Then add baking powder and 150 gm flour. Whisk further.
Now add 100 ml milk and the rest of the 150 gm flour. Whisk and mix well.
Now, remove about 1/3 of this batter and transfer it to another mixing bowl.
In that mixing bowl, add 50 ml milk, cocoa powder and coffee powder. Mix well.
In a making mould, pour in 1/2 of the white batter.
Then put the brown batter.
Now add the rest of the white batter.
Gently form waves with a spatula and level the batter by tapping the mould.
Transfer the mould to the preheated oven and bake the cake for about 55-60 minutes. After 50 minutes, insert a toothpick and check. If it comes out clean, your cake is ready. If it doesn't, bake for some more time. By approximately 60 minutes, the cake should be ready.
Cool on a wire rack. Slice when completeley cooled.