Begin by creaming the softened butter and caster sugar together until the mixture is light and fluffy.
Gradually incorporate the eggs, mixing well after each addition.
Next, add the whole milk, vanilla essence, and self-raising flour, stirring until everything is thoroughly combined.
Divide the batter evenly into two bowls. In one bowl, add the cocoa powder and mix until fully incorporated.
Spoon the two batters into a lined loaf tin, alternating between the plain and chocolate mixtures.
Gently shake the tin to help the batter spread to the edges.
Bake in the oven for 55 minutes.
Once baked, remove the cake from the oven and allow it to cool completely before slicing into portions.
