Black and White Cookies

Preheat and prepare: Preheat oven to 350°F . Line 2 baking sheets with parchment paper.
Whisk the dry ingredients together: In a medium bowl, whisk together 1 3/4 cups flour, 1/2 teaspoon baking powder, 1/4 teaspoon baking soda, and 1/4 teaspoon salt.
Cream wet ingredients together: In a large bowl or stand mixer fitted with a paddle attachment, beat together 8 Tablespoons butter and 1 cup sugar on medium-high speed until smooth and creamy, about 2 minutes. Add the egg and 2 teaspoons vanilla extract, and beat on high speed until combined, about 1 minute. Scrape down the sides of the bowl, as needed.
Add the dry ingredients to the wet ingredients: Reduce to low speed and add 1/2 of the the dry ingredients , then 1/2 of the sour cream. Beat everything on low until combined, then add remaining flour and sour cream. Batter will be thick. Refrigerate dough for 15 minutes.
Drop onto prepared sheet and bake: Using a large cookie scoop, add about 6 cookies per sheet. Bake for 15 minutes or until the edges are lightly golden. Allow the cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before icing.

Make the white icing: In a medium sized bowl, whisk the 5 cups powdered sugar, 6 of the Tablespoons of milk, 2 Tablespoons corn syrup, 1 teaspoon vanilla, and 1/2 teaspoon salt together in a medium bowl. If it's too thick to whisk, you can add an extra Tablespoon of milk.
Make the chocolate icing: Transfer 1 cup of the prepared icing into a separate bowl, add the remaining Tablespoon of milk and the 3 Tablespoons of cocoa powder. Whisk until combined.
Spread icing, rest, and spread again: Spread vanilla icing onto half of each cookie. Refrigerate for 15 minutes or until set so that the icings do not bleed into each other. Spread chocolate icing onto other half and allow the icing to set completely, about 1 hour.