Preheat oven to 350°F (175°C). Line an 8×8-inch pan with parchment paper and lightly grease it.
Melt butter and peanut butter together until smooth. Stir in brown sugar and mix well. Let cool slightly.
Add egg, egg yolk, milk, and vanilla. Whisk until smooth.
Stir in flour, baking powder, and salt until just combined.
Fold in most of the toffee bits and chocolate chunks, saving some for topping.
Press dough evenly into the pan. Sprinkle remaining toffee and chocolate on top.
(Optional) Chill dough for a few hours or overnight for better flavor.
Bake for about 26 minutes, until edges are golden and center is slightly soft.
Sprinkle flaky sea salt on top and let cool completely before slicing.
