Preheat the oven to 350°F (175°C) and coat a bundt pan with baking spray or butter and flour.

In a large bowl, cream together butter, peanut butter, and vanilla until light and fluffy.

Gradually add granulated and brown sugars, mixing until fluffy.

Add eggs one at a time, mixing on low speed after each addition.

Whisk together flour, baking powder, and salt in a separate bowl, then add to the wet mixture alternating with buttermilk.

Fold in mini Reese's cups, milk chocolate chips, and peanut butter chips.

Pour batter into the bundt pan and bake for 1 to 1.5 hours, checking for doneness.

Cool in the bundt pan for 20 minutes, then invert onto a wire rack to cool completely.

For the glaze, microwave chocolate chips, heavy cream, and peanut butter until melted and smooth.

Drizzle over the cooled cake, optionally garnishing with reserved chopped Reese's cups.