Cook butter over medium heat until the milk solids turn golden brown and smell nutty (about 5-7 minutes), then cool completely before mixing into batter to prevent cooking the eggs and preserve that deep, complex flavor.

Mix just until combined after adding flour to avoid developing gluten, which leads to tough, cakey blondies instead of the desired chewy, fudgy texture.

Reserve half your toffee pieces to fold in at the very end and sprinkle on top before baking—this ensures some pieces stay intact and create bursts of toffee flavor rather than dissolving completely into the batter.

Remove blondies when a toothpick inserted in the center still has a few moist crumbs (not fully clean), as carryover cooking will set them to perfect chewy consistency without drying them out.

Sprinkle flaky sea salt on the warm blondies right out of the oven so it adheres to the surface and creates a sweet-salty contrast that enhances the toffee and brown butter flavors.