1. Preheat oven to 350°F (175°C). Lightly butter a 10- to 12-inch cast iron skillet.
2. Melt the butter in a saucepot or directly in the skillet over medium heat. Add chopped chocolate, stirring until smooth, then remove from heat and cool slightly.
3. In a large bowl, whisk together the granulated sugar, brown sugar, and eggs until smooth.
4. Add vanilla extract, cocoa powder, salt, and the cooled butter-chocolate mixture to the eggs. Stir until combined.
5. Fold in the flour gently to avoid overmixing.
6. Optionally fold in chocolate chips and toffee pieces.
7. Pour half of the batter into the skillet, then dollop Nutella on top, keeping a 1/2 inch border from the edges.
8. Pour the remaining batter over the Nutella layer, sprinkle with remaining chocolate chips and toffee if using.
9. Bake for 35 to 40 minutes, or until a skewer inserted comes out with moist dark crumbs.
10. Cool in the skillet until desired temperature before serving.