Begin by preheating your oven to 350 degrees F. Then grease and flour a 10-inch bundt pan. Set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
Then, in a stand mixer (or large bowl and hand mixer) beat butter on a medium speed for one minute. Slowly add the sugar and beat until combined. (About four minutes.) Next, slowly add the eggs, one at a time. Scrape the bowl as necessary. Beat in sour cream and vanilla.
Once incorporated, add the flour mixture and milk alternatively until just combined.
Transfer half of the batter (3 1/2 cups) to a medium bowl. Then, stir in the melted chocolate. Stir the peanut butter into the other half of the batter.
Using a separate spoon for each batter, alternatively drop spoonfuls of the chocolate and peanut butter into your pan. Use a knife or spatula to swirl the batter together. Do not over mix. (Fill your pan 3/4 of the way full. If batter remains, use it in another smaller pan or for cupcakes.)
Bake for 60 minutes or until a toothpick comes out clean. Cool the cake in the pan for 15 minutes. Then, remove from the pan to continue cooling.
In the meantime, make your peanut butter glaze. In a bowl with an electric mixer, combine the powdered sugar, peanut butter, and enough half & half to create a thick drizzle.
When your cake is cooled, drizzle with peanut butter glaze.