Instructions

for the cookies

Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Combine 1¾ cups (8¾ ounces) flour, ½ teaspoon baking powder, ¼ teaspoon baking soda, and ⅛ teaspoon salt in bowl.
Using stand mixer fitted with paddle, beat 10 tablespoons softened unsalted butter and 1 cup (7 ounces) granulated sugar on medium-high speed until pale and fluffy, about 2 minutes. Add 1 large egg and 2 teaspoons vanilla and beat until combined. Reduce speed to low and add flour mixture in 3 additions, alternating with ⅓ cup sour cream in 2 additions (about 2½ tablespoons each), scraping down bowl as needed. Give dough final stir by hand.
Using greased ¼-cup measure, drop cookie dough 3 inches apart onto prepared baking sheets. Bake until edges are lightly browned, 15 to 18 minutes, switching and rotating sheets halfway through baking. Let cookies cool on sheets for 5 minutes, then transfer to wire rack to cool completely, about 1 hour.

for the glaze

Whisk 5 cups (20 ounces) confectioners’ sugar, sifted; 6 tablespoons milk; 2 tablespoons corn syrup; 1 teaspoon vanilla; and ½ teaspoon salt together in bowl until smooth. Transfer 1 cup glaze to small bowl; reserve. Whisk 3 tablespoons Dutch-processed cocoa powder, sifted, and remaining 1 tablespoon milk into remaining glaze until combined.
Working with 1 cookie at a time, spread 1 tablespoon vanilla glaze over half of underside of cookie. Refrigerate until glaze is set, about 15 minutes. Cover other half of cookies with 1 tablespoon chocolate glaze and let cookies sit at room temperature until glaze is firm, at least 1 hour. Serve. (Cookies can be stored at room temperature for up to 2 days.)