Preheat your oven to 350 degrees (F) (175 degrees C). Grease an 8x8 pan; set aside.
Melt butter, then mix with brown sugar until smooth. Beat in the egg, then add the vanilla; stir until well combined. Mix in the salt and flour and stir until everything is combined. Quickly fold in the chocolates. Smooth batter into prepared pan and bake for 20-25 minutes. The center should not shake when you wiggle the pan, but they will be moist and gooey. Cool completely in pan before cutting and serving.
Blondies are extremely versatile! Swap out one mix-in for another if you'd like! I've added butterscotch chips, nuts, dried fruit, and even granola to these blondies and they always turn out great!