Heat the oven to 350 F and spray a bundt pan with non-stick spray.
In the bowl of a mixer cream the butter, vanilla and both sugars until very fluffy, about 3 minutes at medium speed. Add the eggs, one at a time, and blend until they are just mixed.
In a separate bowl whisk together the flour, cocoa powder, baking powder, baking soda, and salt until well mixed.
Alternately add the flour mixture and buttermilk into the butter, in three additions, beginning and ending with the flour. Add the peanut butter chips and mix until evenly distributed. The batter will be thick.
Spoon the batter into the prepared pan and smooth the top, making sure the batter is even. Bake for 50 to 55 minutes, or until the center of the cake springs back when gently pressed and a toothpick inserted into the thickest part of the cake comes out with only a few crumbs clinging to it.
Cool the cake in the pan for ten minutes before turning out onto a wire rack to cool.
While the cake cools prepare the glaze.
In a bowl whisk together the butter and peanut butter until well mixed. Whisk in the powdered sugar and two tablespoons of the milk and whisk until smooth. Add more milk, a few teaspoons at a time, until the glaze is pourable but still fairly thick.
Once the cake has cooled some, but still slightly warm, place a piece of parchment paper under the rack and pour the glaze over the top.