Divide the batter into three portions; put one third (about 1 and 1/3 cups or 300 grams) in the bowl with the melted chocolate and stir them together along with the cocoa powder. Add another third of the batter in the bowl with the dulce de leche and stir until combined. Stir the vanilla extract into the remaining one-third of the batter in the mixing bowl. Be careful not to overmix.
Using an ice cream scoop or 2 tablespoons (using one spoon to scoop up the batter and the other to scrape it off, to release it and drop it into the loaf pan), make five rows of chocolate and vanilla batter side-by-side in an alternating pattern down both sides of the pan (as shown in the post). Add scoops of the dulce de leche batter down the center of the pan.
Working to create a checkerboard, for the next layer, put scoops of chocolate batter over the scoops of vanilla batter in the pan, then put scoops of dulce de leche batter over the scoops of chocolate batter in the pan. Finish (well, almost!) with scoops of vanilla batter down the center of the pan. Place scoops of leftover batters randomly on top. Tap the pan lightly on the counter to settle the batters.
Store cake for 2-3 days in the refrigerator. Serve at room temperature. The cake also freezes well. Thaw overnight in the fridge or for a couple of hours at room temperature.