Preheat oven to 350˚.
Spray or butter 9x13 pan with either coconut oil spray or butter and set aside the pan. See notes below regarding baking time with a metal or glass pan.
Using a stand or a hand mixer, cream the butter and brown sugar together for 2 minutes until light and fluffy.
Add eggs, one at a time, mixing well after each addition, finishing with egg yolk.
Meanwhile, in a medium bowl, mix dry ingredients together and blend using a whisk. Set bowl aside.
Slowly add in dry ingredients to brown sugar mixer beating lightly until incorporated.
Using a wooden spoon or on lowest mixer setting, stir in dark and semi-sweet chocolate chips and toffee pieces.
Plop big spoonfuls blondie dough into prepared pan spacing them throughout the pan evenly.
Using a flat spatula or knife, even out dough in pan.
Sprinkle lightly with the white chocolate chips and a little flaky sea salt.
Bake for approximately 25 minutes checking around the 20-minute mark until golden brown. Don't overbake the bars or they will end up on the cakier side. (see notes below for specific pan baking instructions)
Let cool completely before cutting. Cut into 24 squares.