Preheat oven to 350 degrees. Prepare two large baking sheets by lining with parchment paper. Set aside.

Add flour, baking powder, baking soda, and salt to medium bowl. Whisk together to combine. Set aside.

In a bowl fitted for a stand mixer, beat butter and sugar with a paddle attachment until nice and smooth - about 3 minutes. Add egg and vanilla and beat until just combined. Add 1/3 of flour mixture and beat until combined. Add 1/2 of sour cream and beat until combined. Repeat until flour mixture and sour cream are fully incorporated, ending with the last 1/3 of the flour mixture. Batter should be super thick.

Using an ice cream scoop of 1/4-cup measuring cup, scoop cookie dough and place onto prepared cookie sheet with tons of space between cookies. 6 cookies per sheet. Place in preheated oven and bake for 14-15 minutes, or until the edges turn slightly golden brown. Not too long or they'll get crunchy instead of cakey!

Remove from oven and let cool for 5 minutes before transferring to wire rack to cool completely. Do not ice cookies until completely cool.

Whisk the powdered sugar, 6 tablespoons milk, corn syrup, vanilla extract, and salt in large bowl. Transfer 1 cup of mixture to another bowl. Sift 3 tablespoons of cocoa powder into one bowl and add last remaining tablespoon of milk. Whisk together to make chocolate icing.

To frost cookies, start with white icing. Spread icing with an offset spatula on half of each cookie. Set back on cooled baking sheet and place in fridge to let the icing set up for 15 minutes. Once set, repeat with chocolate frosting. Feel free to go over some of the white icing to create a nice, flat edge in the middle. Place back in fridge to set for another hour. Remove from fridge and let them get back to room temperature before serving. Enjoy!