Preheat oven to 350°F (180°C). Line 2 baking sheets with parchment paper or silicone baking mats.

Whisk the flour, baking powder, baking soda, and salt together in medium bowl. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until smooth and creamy, about 2 minutes. Add the egg and vanilla extract, and beat on high speed until combined, about 1 minute. Scrape down the sides and up the bottom of the bowl and beat again as needed to combine. Reduce to low speed and add the dry ingredients in 3 additions, alternating with the sour cream. Beat everything on low until combined and no pockets of flour remain. Batter is extremely thick.

Using a greased 1/4-cup dry measuring cup, drop mounds of dough 4 inches apart on prepared baking sheets—6 cookies per sheet. Bake for 16-18 minutes or until the edges are lightly browned. Allow cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before icing.

Whisk the confectioners’ sugar, 6 Tablespoons milk, the corn syrup, vanilla extract, and salt together in a medium bowl. Transfer 1 cup to a separate bowl, add remaining Tablespoon of milk and the cocoa powder. Whisk until combined.

Spread vanilla icing onto half of the cookies—the flat side. Refrigerate for 15 minutes or until set so that the icings do not bleed into each other. Spread chocolate icing onto other side and allow the icing to set completely, about 1 hour, before serving.

Cookies will stay fresh in an airtight container at room temperature for 2 days or in the refrigerator for up to 1 week.