Prepare the Oven and Pan: Preheat the oven to 350°F. Coat a 9 × 13-inch baking pan with cooking spray to prepare it for the brownie batter.
Mix the Brownie Batter: Prepare the boxed brownie mix according to the package instructions, adding all the ingredients specified on the box. Stir as directed until the batter is ready.
Fill the Baking Pan: Pour the prepared brownie batter into the greased pan and spread it evenly across the bottom.
Partially Bake the Brownies: Place the pan in the preheated oven and bake for 15 minutes. The brownie base will still be soft at this stage, which is expected before adding the topping.
Prepare the Chocolate Topping: While the brownie base bakes, combine the semi-sweet chocolate chips and sweetened condensed milk in a microwave-safe bowl.
Melt the Chocolate Mixture: Microwave the mixture on high for 60 seconds, stirring halfway through. If the chocolate chips have not melted completely, continue microwaving for a little longer as needed, until the mixture becomes smooth.
Spread the Chocolate Topping: Carefully remove the partially baked brownies from the oven. Drizzle the melted chocolate mixture over the surface, then gently spread it with the back of a spoon to cover the brownie base.
Add the Toffee Bits: Sprinkle the toffee bits evenly over the chocolate topping, distributing them across the surface.
Finish Baking: Return the pan to the oven and bake for another 18–20 minutes, allowing the brownie base and chocolate topping to finish baking together.
Cool and Slice: Remove the brownies from the oven and let them cool completely in the pan before cutting into squares. This allows the layers to set for easier slicing and serving.
