Just look to the cookie.
Cookies:
1 1/4 cups all purpose flour
1/2 tsp baking soda
1/2 tsp coarse salt
6 T unsalted butter, room temperature
1/2 cup granulated sugar
1 large egg
1/2 tsp pure vanilla extract
1/3 cup low-fat buttermilk
Icing:
2 cups confectioners' sugar
1 T plus 1 tsp light corn syrup
2 1/2 tsp fresh lemon juice
1/4 tsp pure vanilla extract
1 T water, plus more if needed
1 T unsweetened Dutch-process cocoa powder
1. Preheat oven to 350. Make cookies: sift together flour, baking soda and salt into a bowl.
2. Put butter in the bowl of an electric mixer fitted with paddle attachment. Mix until creamy, about 2 minutes. Add granulated sugar; mix until fluffy, about 3 minutes. Mix in egg and vanilla. Mix in flour mixture in three batches, alternating with buttermilk.
3. Roll tablespoons of dough into balls; drop onto baking sheets lined with parchment, spacing 2 inches apart. Bake cookies, rotating sheets halfway through, until bottoms turn golden, about 10 minutes. Let cool completely on sheets on wire racks.
4. Make icing: Whisk confectioners' sugar, corn syrup, lemon juice, vanilla, and the water in a small bowl until smooth. Add more water if needed to achieve a consistency slightly thicker than honey. Transfer half the icing to a small bowl. Stir in cocoa powder; thin with water to match white icing consistency.
5. Spread white icing on half of each cookie's flat side and cocoa icing on the other half. Let stand until set, 30 minutes. Cookies can be stored between layers of parchment in airtight container at room temperature up to 3 days. Makes about 4 dozen.
Recipe from Martha Stewart's Cookies