Preheat oven to 350°F. Line 2 cookie sheets with parchment paper or silicone mats.

Whisk flour, baking soda, and salt in a small bowl. Set aside.

In a stand mixer fitted with the paddle attachment or a large bowl with an electric hand mixer, cream butter and granulated sugar on medium-high speed until light and fluffy, about 3 minutes. Scrape down the sides and bottom of the bowl. Beat in eggs and vanilla until smooth.

Running the mixer on low, add half of the flour mixture; mix until just combined. Add all of the sour cream; mix until incorporated. Add the rest of the flour mixture and mix just until combined. The dough will be soft and sticky.

Using a large (3-tablespoon) cookie scoop, drop 3 tablespoons of dough per cookie, 3-inches apart, on prepared cookie sheets. Bake until edges are light golden brown and centers are just set, 15-18 minutes. Let cool completely before frosting.

Once cookies are cool, turn them over, flat side up.

To make the white frosting, whisk together 1 ¼ cups (141g) powdered sugar with 1 tablespoon corn syrup, ½ teaspoon vanilla, and 1-2 tablespoons warm water (add 1 teaspoon at a time) until spreading consistency. Frost half of each cookie with the white frosting.

To make the black frosting, whisk together 1 ½ cups (170g) powdered sugar and the cocoa with 1 tablespoon corn syrup, ½ teaspoon vanilla, and 1-3 tablespoons warm water (add 1 teaspoon at a time) until spreading consistency. Frost the other half of each cookie.

Let cookies sit about 20 minutes to set.

Store in an airtight container between sheets of wax paper at room temperature for about 3 days or freeze for up to 3 months.