Preheat oven to 350 degrees.
To a bowl mix melted butter, brown sugar, granulated sugar, vanilla, eggs and egg yolk.
In a separate bowl sift together flour, baking powder, corn starch and salt.
Slowly add the dry ingredients into the wet.
Mix until fully combined.
Measure one cup of the Heath toffee bits and add to the batter. Fold in.
Line a 9x13 pan with parchment paper.
Spread the thick batter into the pan evenly.
Bake in the middle rack for 35 minutes uncovered.
Remove and allow to cool for 30 minutes before cutting into 15 bars.