Pre-heat the oven to 350 degrees. Butter a 9x13 baking dish.In small saucepan over low heat, melt butter completely. Stir in sugar and continue cooking for 1-2 minutes, stirring constantly. Do not allow sugar mixture to boil.Remove from the heat and stir in the Nutella. Pour butter mixture into a large bowl or stand mixer, beat in cocoa powder, eggs, salt, baking powder, espresso powder, and vanilla extract. Mix until well combined.Stir in the flour and baking melts until just combined.Spread into prepared pan (batter will be very thick and sticky) and bake 30-35 minutes.