Take out about 3-4tbsps of the batter & add cocoa powder. Mix until incorporated. In a lined loaf pan, pour part of the plain batter, then all the chocolate batter, then the remaining plain batter.

Using a knife, make a horizontal motion (side to side) through then a vertical motion (top to bottom). Tap to remove air bubbles. Bake in a preheated oven

Take out, allow to cool & remove the parchment paper from the base

Cut as desired.