Preheat the oven to 350°F. Grease a 9 x 13 pan and line it with parchment paper.
In a large bowl, whisk together the flour and baking powder.
1½ cups all purpose flour, 1½ tsp baking powder
In a saucepan over medium heat, combine the butter, light brown sugar and salt. Allow to melt completely, whisking frequently for about 2-3 minutes. Remove from the heat and add the vanilla. Then allow the mixture to cool for 10-20 minutes until it's around room temperature.
12 tbsp unsalted butter, 1½ cups light brown sugar, ¾ tsp salt, 1 tbsp vanilla extract
Add the egg to the saucepan and whisk it together. Pour the mixture into the bowl with the flour and fold together with a spatula.
1 egg
Pour in the M&Ms and and fold them into the batter. Add the chopped chocolate and gently combine.
⅓ cup mini M&M's, ¼ cup dark chocolate
Pour the batter into your pan and smooth evenly. Place in the oven to bake for 20-25 minutes, until the edges are just golden and knife comes out clean when inserted in the middle. Remove from the oven and allow them to cool completely.
Slice and serve.