Preheat the oven to 350°F/180°C, and line 2 cookie sheets with parchment paper.

In a large bowl, combine flour with baking powder, baking soda, and salt. Set aside.

In a separate bowl, using a hand or a stand mixer fitted with a paddle attachment beat the butter with sugar until smooth and creamy (2-3 minutes).

Add the eggs and vanilla, and beat again until well combined.

Add the dry ingredients in batches alternating with sour cream, beat until combined scraping the sides of the bowl as needed. The cookie dough will be thick.

Using a ¼ cup measuring cup or cookie scoop, drop cookies onto the prepared cookie sheet about 4 inches apart. Bake in the preheated oven for 15-17 minutes, or until the center of the cookie feels springy to the touch.

Allow to cool on the cookie sheet for 5 minutes, then transfer to a wire rack to cool completely.

To make the icing, in a bowl combine the powdered sugar with 2 tablespoons of milk, corn syrup, vanilla, and salt. Whisk until combined, and add more water if needed until you reach a spreadable consistency.

Transfer half of the cookie icing to a separate bowl, and add the cocoa powder plus 1 tablespoon of milk. Whisk and add more milk/water to reach the desired consistency.

Using a small offset spatula, ice the FLAT side of the cookie with a thin layer of icing (only half of the cookie), and chill in the fridge for 15 minutes. You want the icing to be set so it does not bleed into the chocolate icing once applied.

Ice the other half of each cookie with chocolate icing, and allow it to set completely.