Twelve cookies may not sound like much, but these cookies are huge. You'll get neater cookies if you spread on the vanilla glaze first. This recipe provides a little extra glaze, just in case. Makes 12 large cookies
1. For the Cookies: Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees F. Line 2 baking sheets with parchment paper. Combine flour, baking powder, baking soda, and salt in bowl.
2. Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 2 minutes. Add egg and vanilla and beat until combined. Reduce speed to low and add flour mixture in 3 additions, alternating with 2 additions of sour cream, scraping down bowl as needed. Give dough final stir by hand.
3. Using greased 1/4-cup (2-ounce) measure or portion scoop, 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.
4. For the Glaze: Whisk sugar, 6 tablespoons milk, corn syrup, vanilla, and salt together in bowl until smooth. Transfer 1 cup glaze to small bowl; reserve. Whisk cocoa and remaining 1 tablespoon milk into remaining glaze until combined.
5. 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.)