Preheat oven to 170C and line 20cm baking pan with parchment paper
Using a mixer cream butter and sugars together until smooth and fluffy
Add eggs one at a time and mix until smooth and fully incorporated
Fold in flours, xantham gum and baking powder followed by milk chocolate chunks
Spread batter evenly into the prepared pan, smooth out the top and sprinkle with sea salt
Bake for 25-30 minutes until set and slightly golden on top
Allow to cool fully before cutting into squares