Twelve cookies doesn't 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.
To make the cookies:1. Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees.
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 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.
To make the glaze:1. 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.
2. 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.
(Cookies can be stored at room temperature for up to 2 days.)