In a large bowl, cream the butter and brown sugar until smooth and fully combined. Add the egg and mix until incorporated. Add the flour, baking powder, baking soda, and chopped chocolate, mixing just until no dry spots remain.

Press half of the dough into an even layer in the prepared pan. Spread a thick, even layer of Nutella and Kinder over the dough. Freeze until firm, about 10 minutes.

Cover the filling with the remaining dough, gently sealing the edges. Bake until the surface is set and lightly golden, exactly 35 minutes.

Cool completely in the pan before slicing and serving.