Preheat your oven to 350°F. Generously grease a 12-cup muffin tin with cooking spray. Using a muffin tin is the secret to getting those perfectly uniform, handheld layers.

Divide the cookie dough into 12 even pieces. Press one piece into the bottom of each muffin cup, creating a flat, even base.

Sprinkle a teaspoon of toffee bits directly onto the raw cookie dough. This ensures the crunch is "locked in" between the cookie and the candy.

Place one peanut butter cup upside down on top of the cookie dough in each well. Placing it upside down provides a wider, flatter surface for the brownie batter to rest on, preventing it from sliding off during the bake.

Pour the prepared brownie batter over the peanut butter cups. Fill each well until it is about 75% to 80% full. Do not overfill, or the brookies will "mushroom" over the edges of the tin.

Bake for 20–25 minutes. You want the brownie tops to be set and slightly cracked but still fudgy in the center. Immediately after removing them from the oven, sprinkle a pinch of flaky sea salt over each cup.

Allow the brookies to cool in the pan for at least 10–15 minutes. If you try to remove them while hot, the melted peanut butter center will cause the layers to separate.