Step 1: Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: Line a 9x13 inch baking pan with parchment paper, leaving some overhang on the sides.
Step 3: In a mixing bowl, combine the softened butter and light brown sugar. Cream them together until light and fluffy.
Step 4: Add the egg and vanilla extract, mixing well until fully combined.
Step 5: In a separate bowl, whisk together flour and salt. Gradually add to the wet ingredients until just combined.
Step 6: Fold in 1 cup of semi-sweet chocolate chips and 1 cup of peanut butter chips.
Step 7: Pour the batter into the prepared pan and spread it out evenly.
Step 8: Drizzle sweetened condensed milk over the batter and swirl it in gently.
Step 9: Top with remaining toffee bits, 1 cup of semi-sweet chocolate chips, and 1 cup of peanut butter chips, pressing them down gently.
Step 10: Bake for 25-30 minutes or until edges are golden. Cool completely before cutting into squares.