Cream together the softened butter and light brown sugar in a bowl.

Add the egg and vanilla extract, then mix in the flour and bicarbonate of soda until a dough forms.

Fold in the chocolate chips and M&M’s.

Take a third of the dough to form a lid and press the rest into the cake tin, creating a lip around the edge.

Dollop Nutella into the center, smoothing it evenly. Place the dough lid on top and press down to seal the edges.