Step 1

Brown the butter: Cut the butter into small chunks and transfer them to a medium saucepan. Warm over medium-low heat, stirring constantly, until the butter foams, smells nutty, and brown bits form on the bottom, about 5-7 minutes. Pour into a large heatproof bowl (with the brown bits) and let cool for 5 minutes.

Step 2

Make the cookie dough: Add the brown sugar and granulated sugar to the cooled butter and beat with an electric mixer until creamy, about 2 minutes. Beat in the egg, egg yolks, and vanilla until smooth.

Step 3

In a separate bowl, whisk the all-purpose flour, baking soda, and salt. Gradually stir the dry ingredients into the wet ingredients, stopping when about halfway combined (there should be streaks of flour visible).

Step 4

Add the chocolate and toffee bits and stir until combined. It may seem crumbly, but keep stirring until it’s thick and shiny.

Step 5

Portion and rest: Portion the dough into 1-2 tablespoon balls on a parchment-lined baking sheet. Roll between your hands to get them smooth, then wrap the tray tightly with plastic and chill the dough in the fridge for at least 2 hours or up to 3 days.

Step 6

When ready to bake, preheat the oven to 350°F and line a second baking sheet with parchment paper.

Step 7

Divide the cookies between the baking trays, leaving 2-3 inches between each ball. Sprinkle with flaky sea salt and bake for 10-12 minutes, until the edges are golden brown but the centers are still soft. Let the cookies cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.

Step 8

Optional: Press a few extra chocolate chunks and toffee pieces into the tops of the warm cookies just after baking.

Step 9

Serve warm or at room temperature, and enjoy!