Melt Butter in a sauce pan. Transfer to a large bowl, let it cool down.
Preheat oven. Line and flour or spray a 8x8 square pan or a quarter sheet pan 9 x 13
In a medium bowl, whisk the flour, corn starch and salt. Set it aside. This is your flour mix
Now to the cooled butter, add the brown sugar, beat well until it is blended well. Make sure there are no sugar lumps left.
Then add the eggs and vanilla. Giving a mix, until its just mixed. leaving no streaks
Fold in the flour mix. You will mix, until there are no flour seen.The batter will be gooey
Divide the batter, or just eye ball. First pour the blondie batter, just enough to cover the base of the pan. Fill the next layer with chocolate squares. Cover with the remaining batter. Top it with chocolate chunks.
Bake at 170°c or 350°F. for 25 mins. Until a toothpick inserted comes out with crumbs
Let it sit in the pan for 10 mins, before transfering on to the wire rack.
Let it completely cool down before cutting into bars