Preheat the oven to 350°F. Spray a nonstick 13x9 inch metal pan with cooking or baking spray. Set aside.
In a double boiler or in a heat proof bowl over 1 inch simmering water, melt together butter and 1 cup chocolate chips. Melt until completely smooth.
In a separate bowl, stir together sugar, eggs, corn syrup and vanilla by hand.
Temper the sugar-egg mixture with 1/4 cup warm melted chocolate. While stirring slowly add the remaining melted chocolate and butter. Mix well then set aside to cool for 5 minutes.
Meanwhile, In a separate mixing bowl use a whisk to sift together flour, cocoa powder, baking soda and salt.
Add chocolate mixture to the sifted dry ingredients. Mix by hand until fully moistened. Mix in remaining chocolate chips, 1 cup Reese's pieces and peanut butter cups by hand.
Spread batter evenly into pan. Gently press remaining Reese's pieces on top.
Place into the oven to bake for 25- 30 minutes or until a toothpick inserted into the center shows moist crumbs. Do not over bake.
Cool in the pan on a cooling rack for at least 2 hours before cutting into squares for serving.
Store in an airtight container at room temperature.