Preheat oven to 350°F and grease a bundt pan with cooking spray.
In a large bowl, combine cake mix, instant pudding, eggs, oil, water, and melted butter. Beat with a hand mixer until smooth.
Pour batter into prepared pan and bake for about 45 minutes, until a toothpick comes out clean.
Let cake cool in pan for 10 minutes, then invert onto a cooling rack to cool completely.
Place cake back in the Bundt pan. Scoop out 6 to 8 large holes around the bottom of the cake with a teaspoon, avoiding the top.
Make the filling: Mix butter and peanut butter until combined. Add powdered sugar and heavy cream, beat until fluffy. Transfer to a piping bag.
Pipe filling into holes at the bottom of the cake, then invert onto a serving platter.
Drizzle with melted chocolate and peanut butter, then cover with chopped Reese’s.
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