Preheat the oven to 350°F. Lightly grease two 9-inch round baking pans. Set aside.
In a large bowl, mix together the white sugar, flour, baking soda and salt. Set aside.
In a small saucepan, melt the butter over medium heat. Whisk in the cocoa powder and the hot water, and bring the mixture to a boil. Remove from heat.
Mix in the chocolate mixture into the dry ingredients and mix until fully combined. Add in the eggs one at a time,and beat into the mixture. Add in the milk and 1 teaspoon of vanilla extract, and mix in until fully incorporated.
Pour the mixture evenly into the baking pans, and place in the preheated oven for 30 minutes, or until a toothpick comes out clean.
Meanwhile, in the bowl of a stand mixer, beat together the peanut butter glaze ingredients until fully combined.
Remove the cake from the oven and allow to cool in the pans for 10 minutes, then remove and place on wire racks to cool completely.
Level the cakes (if necessary), then frost the round that you will be using for your bottom, using around 1/3 of the glaze. Place the other round cake on top. Pour the remaining frosting over the cake. This might get messy, so I suggest doing this on your wire racks with a piece of wax paper underneath for easy cleanup.
Roughly chop the reeses peanut butter cups, and sprinkle them over the cake.