Add the sugar, flour, salt and baking soda to a large mixing bowl.
In a saucepan, melt the butter, then add the cocoa and water. Bring to a boil, stirring frequently.
Pour the chocolate mixture over the dry ingredients and stir to combine.
Add the eggs, buttermilk and vanilla; mix until fully combined.
Pour into a greased jellyroll pan.**
Bake for 25 minutes. While the cake is in the oven, prepare the two frostings.
Make the chocolate frosting:
Combine the butter, milk and cocoa powder in a small saucepan. Bring to a boil, stirring constantly. Remove from heat and whisk in the powdered sugar and vanilla. Whisk until smooth.
Make the peanut butter frosting:
In a medium bowl, combine the peanut butter and powdered sugar as much as you can. Add milk slowly (tablespoon by tablespoon) until smooth and creamy.
Top the cake!!
Pour the chocolate frosting over the hot cake. Add large drops of the peanut butter frosting on top. Using the back of a spoon, swirl the two frostings together.
Sprinkle the Reese's Pieces on top and enjoy!!
Notes
*If you don't have buttermilk, add 1 tablespoon vinegar to a liquid measuring cup. Add milk until it reaches the one cup mark. Use this as your buttermilk!**A jellyroll pan is 10.5 x 15.5 inches and will bake for about 25 minutes. If you do not have a jellyroll pan, you can use a sheet pan (13 x 18 inches) and bake for approximately 20-22 minutes.