Place brown sugar in a large metal mixing bowl. Heat butter in a large saucepan over medium heat until boiling. Continue to heat the butter, swirling occasionally, until the sediments (solids at the bottom of the pan) turn golden brown. Remove from heat and pour the butter over the brown sugar into the mixing bowl. Stir butter and brown sugar together and set aside to cool to room temperature (about 20 minutes).

While the butter-brown sugar mixture is cooling, lightly butter 9x13-inch (25x40 cm) baking pan, preheat the oven to 350°F (165°C) and get your other ingredients ready.

When the butter-brown sugar mixture has cooled, stir in the eggs and vanilla. Stir in flour, baking powder and salt. Stir in chocolate chunks and M&Ms.

Spread in prepared baking pan and bake for 20-25 minutes, until the top is lightly browned. Cool and cut into bars. Store in an airtight container at room temperature for up to one week.