Preheat your oven to 350 degrees Fahrenheit and grease a 9×13-inch baking pan. This step is crucial as it prevents the bars from sticking. You want to ensure they come out easily after baking.
In a saucepan over medium heat, melt butter and brown sugar. Stir them together until the mixture is smooth. Being patient during this step is key, as you want the brown sugar to dissolve completely.
Once it’s smooth, take the saucepan off the heat and stir in the peanut butter. Make sure to mix until it’s completely smooth again. This will create a luscious base for your bars.
Let the mixture cool for a few minutes. While it’s cooling, move to a large bowl and whisk together the flour, baking powder, baking soda, and salt. This will ensure the dry ingredients are evenly distributed.
Once cooled, whisk in the eggs, one at a time, into the peanut butter mixture. After this, stir in the vanilla extract. This mixture will be very creamy.
Next, carefully add the peanut butter mixture to the dry ingredients. Stir until just blended; you don’t want to overmix. The batter will be thick but should come together beautifully.
Now, fold in the Reese’s Pieces candies. This adds a great texture and a pop of color to your batter.
Pour the batter into your prepared baking pan, smoothing it out evenly. This ensures every piece bakes uniformly.
Bake in the preheated oven for 25 to 30 minutes, or until the middle is set, and the edges are golden brown. Keep an eye on it; you want a slight jiggle in the center.
While the bars are baking, create the frosting. In a saucepan, combine butter, milk, and granulated sugar. Bring this mixture to a boil, stirring continuously. It’s important to keep it moving to avoid burning.
Let it boil for 30 seconds. This is crucial for achieving the right consistency, so don’t skip this step! Remove from heat, then stir in the chocolate chips and vanilla extract until smooth. You’ll have a rich, glossy frosting!
Once the bars are done baking, pour the warm frosting over them while they’re still in the pan. This allows the frosting to seep into the bars a bit, enhancing the flavor.
Finally, sprinkle some extra Reese’s Pieces on top for garnish. Let the bars cool completely in the pan before slicing into squares.