In a medium-sized bowl, combine graham cracker crumbs and powdered sugar until well blended. Set aside.In a microwave-safe measuring cup, heat 6 Tbsp butter and peanut butter together in microwave for 15 second increments until melted. Stir until combined and pour over graham cracker mixture. Continue to stir until well combined. Set aside.
Melt half of the chocolate chips with 2 tablespoons of butter in a microwave safe bowl in the microwave for 15 second increments (stirring each time) until chocolate is melted. Place cupcake liners in cupcake tin (mini or regular depending on what size peanut butter cups you want to make-- makes about 36 mini cups or 18 full-sized cups).
Using a paint brush (I used a flat brush about 1/2" wide), "paint" each cupcake liner with melted chocolate. Be sure to evenly coat the bottom and half way up each cup (if using the "mini" cupcake liners). If you are using full-size cupcake liners paint about 1/3 of the way up each cup. Tip: You don't need to paint the chocolate super thick, just enough so you don't see any of the liner underneath.Place in the refrigerator to cool for about 5 minutes or until chocolate hardens.
Press about 1 teaspoon of graham cracker mixture in each mini cup (about 1 Tbsp. for regular-sized cups) making sure the top is somewhat flat (it doesn't need to be perfect). Just make sure you don't go above where you "painted" the chocolate on the cup.
Add remaining chocolate chips to remaining melted chocolate in the microwave-safe bowl and melt together with remaining 2 tablespoons butter (stirring every 15 seconds).Spoon a little chocolate over each peanut butter cup until the peanut butter filling is completely covered (you can lightly shake or tap the cupcake pan to even out the chocolate over the filling). You want the top of the peanut butter cup to look completely flat.
Place in refrigerator or cool area until chocolate hardens, then serve!