Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line with parchment paper, leaving an overhang on the sides.
Step 2: In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, and salt. Set aside.
Step 3: In a large mixing bowl, combine the melted butter, granulated sugar, and brown sugar. Mix until well incorporated.
Step 4: Add the eggs one at a time to the butter-sugar mixture, beating well after each addition. Stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Do not overmix.
Step 6: Fold in the chocolate chips and the sweetened condensed milk until evenly distributed.
Step 7: Pour the batter into the prepared baking pan and spread evenly.
Step 8: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs. The edges should be set, but the center might still be slightly soft.
Step 9: Remove from the oven and let cool completely on a wire rack before cutting into squares and serving.