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.
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.)
Peanut Butter Glaze: