Preheat oven to 180 deg c. Line a loaf tin with baking spray and parchment paper.
Melt the dark chocolate in a mixing bowl and set aside to cool.
In another large bowl add the butter and flour, rub together with your fingers until it resembles damp sand
Add the sugar, vanilla, eggs, baking powder, baking soda, salt and milk. Whisk until smooth.
Divide the batter into half, place half the batter into the melted chocolate bowl. Add the cocoa powder, coffee powder, milk and whisk together.
Spoon the vanilla batter and chocolate batter into the lined loaf tin, alternating. Continue until all the batter is finished.
Using a butter knife swirl the batters together and bake for 28-30 mins until risen and tooth pick inserted into the center comes out clean.
Let it cool before slicing