Step 1

Preheat oven to 350 degrees F (180°C).

Step 2

Line 2 cookie sheets with parchment paper and set them aside.

Step 3

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

Step 4

In a large bowl using a mixer fitted with a paddle attachment, beat together the butter and sugar on medium-high speed until smooth and creamy, about 2 minutes.

Step 5

Add the egg and vanilla extract, and beat until combined, about 1 minute. Scrape down the sides and bottom of the bowl and beat again.

Step 6

Reduce the speed to low and add the dry ingredients in 3 additions, alternating with the sour cream after each addition. Beat until fully combined, the batter will be extremely thick.

Step 7

Using a greased 1/4-cup measuring cup or cookie scoop, drop mounds of dough 4 inches apart on the prepared baking sheets, which is about 6 cookies per sheet.

Step 8

Bake for 16-18 minutes or until the edges are lightly browned.

Step 9

Allow the cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before adding the icing.

Step 10

Whisk the powdered sugar, 6 tablespoons of milk, the corn syrup, vanilla extract, and salt together in a medium bowl.

Step 11

Transfer 1 cup of the mixture to a separate bowl, add the remaining milk and the cocoa powder. Whisk until combined.

Step 12

Spread vanilla icing onto half of the cookies. Refrigerate for 15 minutes or until set. After that, spread the chocolate icing onto the other half and allow the icing to set completely, about 1-2 hours, before serving.