In a large saucepan over medium heat, melt the butter. Turn off the heat and add the chocolate. Whisk until the chocolate is melted.
3
Whisk in the sugar, vanilla and egg until smooth. Stir in the flour until well combined. Pour the batter into a large rimmed baking sheet and tilt back and forth, allowing the batter to flow into a thin, even layer across the pan. Scatter the peanut butter morsels (or topping of your choice) evenly over the top.
4
Bake for 10 minutes or until brownie is just set. Remove from oven and cool on a wire rack for 30 minutes.
5
Break into large rough-shaped pieces, as you would a nut brittle.