Select two microwave safe mugs. In each mug place one tbsp of butter and 1 oz of semisweet chocolate. Microwave each mug for 40 seconds (microwave them separately). Remove and stir until chocolate is completely melted with a small fork.
Add 2 tbsp sugar to each mug and stir. Beat egg in a small bowl, add half to each mug and mix. Add in 1 tbsp flour to each cup and mix until smooth. Add in 1 1/2 tablespoons of Nutella into the cup, trying to center it in the middle of the batter (you don't need to squish down as the Nutella will start to sink and the cake will rise over it when baking.
Cook each brownie for 1 minute in microwave (do each separately). Serve while warm.