Preheat oven to 350℉ and line two large baking sheets with parchment paper. Whisk together flour, baking powder and soda, and salt and set aside.
In a stand mixer with the paddle attachment, beat sugar and butter together on medium speed until fluffy, about 3 minutes. Then add vanilla and egg and mix until incorporated. Reduce to low speed and alternate adding 1/3 of flour mixture, 1/2 the sour cream, then 1/3 of flour mixture, then remaining sour cream and then remaining flour mixture. Allowing ingredients to mix together between each addition. Use a rubber spatula to scrape down sides of bowl as needed during process. Finally give everything a stir all together by hand, scraping bottom of bowl, to ensure no dry pockets remain.
Drop dough in 2 tbsp portions onto baking sheets with plenty of room to spread. Bake on racks in middle of oven, rotating half way through baking, to ensure even baking, for 12 minutes until edges are set and just barely light golden brown. Allow cookies to cool for 5 minutes on baking sheets then transfer to wire rack to cool completely.
To make icing you will need two bowls. Begin by making the vanilla icing. Whisk powdered sugar, 3 tbsp of milk, corn syrup, vanilla and salt together until smooth. In a separate bowl, whisk cocoa powder with 1/2 tsp of milk, then add the vanilla icing and mix until smooth.
To ice cookies, ice half of each cookie with vanilla and allow to dry completely or place in fridge for 15 minutes to expedite process. Then ice other half with chocolate and allow to dry completely, about 1 hour. Store cookies covered for up to a week.