Preheat oven to 350 degrees F. Line a 9"x9" square baking pan with 2 pieces of foil (foil needs to be large enough to hang over the sides), then grease the foil with butter or nonstick cooking spray. Set aside.

Whisk together the flour, salt and baking powder in a small mixing bowl. Set aside.

Melt the butter, chocolate chips and cocoa powder in a large saucepan/dutch oven over medium-low heat, stirring constantly until smooth and completely combined/melted. Do not let the chocolate scorch on the bottom of the pan because scorched chocolate does not taste good.

Once everything is melted together, remove the pan from the heat and stir in the sugar.
Note: The mixture will be grainy. It's ok, it's supposed to be like this.*

Stir in the eggs and vanilla, stirring quickly to prevent the eggs from cooking, then gently stir in the dry ingredients, mixing just until incorporated (do not over-mix).

Pour into the prepared pan, then sprinkle the chopped Reese's peanut butter cups evenly over the top and press down lightly to press them into the batter a bit.

Bake in the preheated oven for 25-30 minutes or until toothpick inserted into the center comes out almost completely clean. (I find brownies to be fickle and the cooking time tends to vary from oven to oven, so start watching them after the 25 minute mark). *These brownies taste best when they are not over-baked. For me, they are always done exactly at the 25 minute mark.*

Remove from oven and allow to cool completely before frosting. While the brownies are cooling, make the frosting.

Place the sugar, peanut butter, butter, vanilla, and salt in the bowl of an electric mixer fitted with a paddle attachment.

Mix on medium-low speed until creamy, scraping down the bowl with a rubber spatula as you work.

Add the cream and beat on high speed until the mixture is light and smooth.

Chop the chocolate into small pieces and place in a bowl.

Heat the cream just until simmering (not quite to a boil yet) and pour over the chocolate.

Allow to sit for a few minutes, then stir until the chocolate is melted.

Cool to room temperature (should only take a couple of minutes), then move on to decorating the brownies.

With the brownies still in the pan, spread the peanut butter frosting over top of the brownies in an even layer, then pour the chocolate ganache over top of the frosting and gently spread out evenly over the top.

Scatter the Reese's peanut butter cup pieces over top of the ganache. They should sink in a bit on their own, but also press them down a little bit so they go down a little into the ganache.

Refrigerate brownies for at least 2 hours before cutting (this allows the frosting and ganache to chill so cutting will be easier and you will have cleaner, prettier brownie pieces when you do cut them).

When brownies are done chilling, lift them out of the pan (lift by the edges of the foil), peel the foil down off the sides and then cut the brownies into ~16 small squares.

Store the brownies in an air-tight container in the refrigerator and remove the desired number of servings about 15-20 minutes prior to serving or microwave for about 8 seconds (much longer than that and the chocolate ganache will completely melt off the top).