Preheat oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper or lightly grease.

• Make the fudge ripple:
  In a small saucepan over low heat, combine sweetened condensed milk, chocolate chips, and butter. Stir until melted and smooth. Remove from heat and stir in vanilla. Set aside to cool slightly.

• Prepare the brownie batter:
  In a medium bowl, whisk together melted butter and sugar until smooth. Add eggs and vanilla, whisking until well combined. Stir in cocoa powder, flour, salt, and baking powder until just mixed. Fold in chocolate chips if using.

• Assemble the brownies:
  Pour half of the brownie batter into the prepared pan and spread evenly. Drizzle half of the fudge ripple over the batter. Add the remaining brownie batter, then drizzle the rest of the fudge ripple on top. Use a knife to gently swirl the layers.

• Bake:
  Bake for 30–35 minutes, or until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs (not wet batter).

• Cool and serve:
  Let cool completely in the pan before slicing. For cleaner cuts, chill in the fridge for 30 minutes before cutting.