- 1 and 3/4 cups (220g) all-purpose flour (or more as needed, see note)
- 1/2 teaspoon baking powder
- 1/4 teaspoon baking soda
- 1/4 teaspoon salt
- 10 Tablespoons (145g) unsalted butter, softened to room temperature
- 1 cup (200g) granulated sugar
- 1 large egg, at room temperature
- 2 teaspoons pure vanilla extract
- 1/3 cup (80g) full-fat sour cream, at room temperature*
- 5 and 1/2 cups (660g) confectioners' sugar, sifted (measure before sifting)
- 7 Tablespoons (105ml) whole milk, divided
- 2 Tablespoons light corn syrup
- 1 teaspoon pure vanilla extract
- 1/2 teaspoon salt
- 3 Tablespoons (18g) unsweetened natural or dutch-process cocoa powder
- Preheat oven to 350°F (180°C). Line 2 baking sheets with parchment paper or silicone baking mats.
- Whisk the flour, baking powder, baking soda, and salt together in medium bowl. Set aside.
- In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until smooth and creamy, about 2 minutes. Add the egg and vanilla extract, and beat on high speed until combined, about 1 minute. Scrape down the sides and up the bottom of the bowl and beat again as needed to combine. Reduce to low speed and add the dry ingredients in 3 additions, alternating with the sour cream. Beat everything on low until combined and no pockets of flour remain. Batter is extremely thick.
- Using a greased 1/4-cup dry measuring cup, drop mounds of dough 4 inches apart on prepared baking sheets-- 6 cookies per sheet. Bake for 16-18 minutes or until the edges are lightly browned. Allow cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before icing.
- Make the icings: Whisk the confectioners' sugar, 6 Tablespoons milk, the corn syrup, vanilla extract, and salt together in a medium bowl. Transfer 1 cup to a separate bowl, add remaining Tablespoon of milk and the cocoa powder. Whisk until combined.
- Spread vanilla icing onto half of the cookies-- the flat side. Refrigerate for 15 minutes or until set so that the icings do not bleed into each other. Spread chocolate icing onto other side and allow the icing to set completely, about 1 hour, before serving.
- Cookies will stay fresh in an airtight container at room temperature for 2 days or in the refrigerator for up to 1 week.
Make ahead tip: You'll want to bake this cookie dough right away as the baking powder is initially activated once wet. You can, however, bake the cookies in step 4, cool completely, cover tightly, and store at room temperature for up to 3 days before continuing with step 5. Frosted or unfrosted cookies freeze well for up to 3 months. Thaw overnight in the refrigerator and bring to room temperature before serving or frosting.