Butter a 8 by 8” Square pan.
In a double boiler, melt butter completely. Stir in sugar and continue cooking for 1-2 minutes, stirring constantly. Do not allow the mixture to boil.
Remove from the heat and stir in the Nutella until well combined.
Beat in cocoa powder, eggs, salt, baking powder, and vanilla extract. Mix until well combined.
Stir in the flour and Choc until just combined.
Spread into prepared pan (batter will be very thick and sticky) and bake for about 25-30 minutes, until a tester comes out mostly clean. The edges should be set and the center should still look slightly moist, but not uncooked. Cool on a wire rack.