In the bowl of a stand mixer
, beat the butter on high speed until creamy, about 1 minute. Add brown sugar and cream for another 2-3 minutes or until light and fluffy. Scrape sides of mixing bowl. At medium-high speed, add eggs one at a time, mixing for about 30 seconds in between each addition. Scrape sides of bowl again then add sour cream and vanilla and beat for another 30 seconds or until incorporated. With the mixer on low speed, add ⅓ of the flour mixture, then add half of the chocolate mixture and mix to combine. Repeat ending with the flour mixture being careful not to overbeat. Remove the bowl from the mixer, scrape sides and mix well by hand until incorporated.