In a medium bowl using an electric mixer on low speed, beat together the cake mix, melted butter, water, and eggs for 30 seconds. Increase speed to medium and beat for 2 minutes.
Grease a 6-quart slow cooker and pour cake batter into it. Add the mini Reese's Cups and stir to combine.
Cook on High for 2 hours or on Low for 4 hours. Turn off the heat immediately after cooking.
In a small microwave-safe bowl, melt 1 cup of peanut butter on HIGH heat for 30 seconds. Stir in powdered sugar.
Drizzle peanut butter icing all over the top of the cake in the slow cooker. Use the back of a spoon to spread it to the edges.
Cut Reese's Miniatures into quarters. Sprinkle over the peanut butter frosting.
Serve warm with ice cream or whipped cream.