Grease a 8" x 8" casserole dish and set aside.

In a large bowl, beat together the butter and sugar until combined.

Add the eggs and vanilla extract and beat until combined.

In a medium-sized bowl, stir together the cocoa powder, flour, salt and baking powder.

Beat the flour mixture into the sugar mixture and then add the peanut butter chips.

Fold the brownie batter out into your prepared casserole dish.

Melt the peanut butter for 15 seconds in your microwave to make it a bit more liquidy, then spoon overtop of the brownies.

Use a butter knife to swirl the peanut butter throughout the brownie batter.

Bake for 25-28 minutes, until the edges of the brownies are crisp and start to pull away from the pan and an inserted toothpick comes out with just a few crumbs and no wet batter.

Allow the brownies to cool completely before frosting.

While the brownies are cooling, prepare your peanut butter frosting.

In a large saucepan over medium-high heat, combine the sugar, milk, butter, shortening and salt.

Stir constantly until the mixture reaches a boil, then allow to boil for 2-3 minutes without stirring.

Remove the saucepan from heat and stir in the vanilla and peanut butter.

Pour the frosting over the brownies and then scatter remaining chocolate chips over the hot frosting.

Allow the chocolate chips to melt slightly for about a minute, and then use a butter knife to swirl the chocolate chips into the frosting.

Let set for 2 hours or overnight before cutting and serving.