Step 1

Preheat the oven to 350°F (180°C). Line 2 baking sheets with parchment paper and grease with nonstick spray.

Step 2

In a large bowl, cream together the butter and granulated sugar with an electric hand mixer on medium-low speed until light and fluffy, 3–4 minutes, increasing to medium speed if necessary. Scrape down the sides of the bowl, then add the egg and vanilla and mix until combined, 1–2 minutes. Add half (2 tablespoons plus 2 teaspoons) of the sour cream and mix until combined.

Step 3

In a medium bowl, whisk together the all-purpose and cake flours, baking powder, baking soda, and salt.

Step 4

Alternate adding half of the flour mixture and the remaining sour cream to the butter mixture, beginning and ending with the flour and beating on low speed between each addition until just incorporated.

Step 5

Using a rounded tablespoon or level #40 cookie scoop, scoop the cookies onto the prepared baking sheets, spacing about 1 inch apart. Dip your fingers in water or lightly spray with nonstick spray and gently flatten the tops of the cookies (this will help prevent them from doming while baking).

Step 6

Bake the cookies for 8–10 minutes, until light golden brown around the edges. Remove from the oven and let cool completely.

Step 7

While the cookies cool, make the icings: In a medium bowl, stir together the powdered sugar, corn syrup, lemon juice, vanilla, and milk until smooth.

Step 8

Transfer half of the icing to a separate medium bowl and stir in the cocoa powder until smooth. Add water, ½ teaspoon at a time, until the chocolate icing has the same consistency as the white icing.

Step 9

Once cookies have cooled completely, flip them over so the flat bottoms are facing up. Using an offset spatula or flat knife, spread a thin layer of white icing over half of a cookie and a thin layer of chocolate icing over the other half. Place on a wire rack, icing side up, and repeat with the remaining cookies. Let the icing set for at least 15 minutes before serving.

Step 10

Enjoy!