Step 1: Prepare the Brownie Batter

1. Preheat your oven to 350°F (175°C) and grease an 8×8-inch baking pan or line it with parchment paper for easy removal.
2. In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add the eggs and vanilla extract, then whisk until the mixture is light and creamy.
3. In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing.
4. Gently stir in the chocolate chips. Set the batter aside.

Step 2: Prepare the Butterscotch Sauce

1. In a small saucepan, melt the butter over medium heat. Add the brown sugar and stir until it dissolves.
2. Slowly pour in the heavy cream while stirring continuously. Let the mixture simmer for 2-3 minutes until slightly thickened. Remove from heat and stir in the vanilla extract. Let the sauce cool slightly.

Step 3: Layer the Brownies

1. Pour half of the brownie batter into the prepared baking pan and spread it evenly.
2. Drizzle the butterscotch sauce over the brownie layer, reserving about 2 tablespoons for topping. Use a toothpick or knife to gently swirl the sauce into the batter.
3. Spread the remaining brownie batter over the top, covering the butterscotch layer. Drizzle the reserved butterscotch sauce over the top and swirl lightly.

Step 4: Bake

1. Place the pan in the preheated oven and bake for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
2. Remove the brownies from the oven and let them cool in the pan for 20 minutes before transferring them to a wire rack.

Step 5: Finish and Serve

1. If desired, melt the remaining chocolate chips in a microwave-safe bowl and drizzle the melted chocolate over the cooled brownies.
2. Sprinkle the flaky sea salt generously over the top. Cut into 16 squares and serve!