Preheat the oven to 180°C/Fan 160°C/Gas 4 and lightly grease a small skillet or baking dish. Melt the butter in the microwave or in a small saucepan. In a bowl, beat together the sugar and egg until combined. Pour in the melted butter and stir until smooth. Mix in the flour, baking powder and salt before stirring in the milk.
In a separate bowl, dissolve the cocoa powder in the hot water to make a thick paste. Add half of the batter mixture to this cocoa bowl and mix until evenly combined. Stir in the chocolate chunks. Stir the dulce de leche into the other half of the batter.
Add alternating spoonfuls of the batter mixtures into your skillet or dish. Drizzle about a teaspoon of extra dulce de leche on top. Use a skewer to gently swirl the batter a little – don’t swirl it too much or the batters will just blend together.
Bake for 17–25 minutes, then remove from the oven and let it cool for a few minutes before diving in.