- 1 stick (4 ounces, 113 grams) unsalted butter
- 3/4 cup dark brown sugar
- 1 large egg plus 1 egg yolk
- 1 teaspoon vanilla extract
- 1/4 teaspoon fine salt
- 1 cup (4.5 ounces, 128 grams) all-purpose flour
- 1/3 cup milk chocolate chips, plus more for sprinkling
- 1/3 cup white chocolate chips, plus more for sprinkling
- 1/3 cup semisweet chocolate chips, plus more for sprinkling
Preheat the oven to 350°F. Line an 8 by 8-inch pan with aluminum foil.
In a small saucepan set over medium heat, melt 6 tablespoons of the butter. Swirling the pan occasionally, continue to cook the butter. It should become foamy with audible cracking and popping noises. Once the crackling stops, continue to swirl the pan until the butter develops a nutty aroma and brown bits start to form at the bottom. Once the bits are amber in color, about 2 to 3 minutes after the popping stops, remove from heat and pour into a mixing bowl. Stir in the remaining 2 tablespoons of butter in until melted.
Combine the browned butter with the sugar and stir until smooth. If still very hot, let cool slightly. Stir in the egg, egg yolk, and vanilla until smooth. Add in the salt and flour and stir until combined. Mix in all the chocolate chips.
Pour the batter into the prepared pan. Sprinkle with a small handful of the three chocolate chips and press into the batter. Bake for about 25 minutes, or until golden at the edges.
Let cool on a baking rack before cutting into squares. Store in an airtight container at room temperature for up to 1 week, or in the freezer for up to 6 months.