2 cups all-purpose flour
1 tsp baking powder
1/2 tsp baking soda
1/8 tsp salt
1/2 cup unsalted butter, softened
1 cup granulated sugar
1 large egg
2 tsp vanilla extract
1/2 cup sour cream
1 cup confectioners' sugar
3 1/2 tbsp heavy cream
1/8 tsp vanilla extract
3 oz semisweet chocolate, coarsely chopped
1/4 cup heavy cream
1 tbsp light corn syrup
Make the cookies
Position a rack in the centre of the oven and preheat the oven to 350 F. Grease two large baking sheets.
In a medium bowl, gently whisk together the flour, baking powder, baking soda and salt. Set aside.
In an electric mixer, using the paddle attachment, beat the butter at medium speed until creamy, about 30 seconds. Gradually beat in the sugar, mixing well until combined. Add the egg and vanilla extract and mix until blended. Scrape down the sides of the bowl. Alternately add the sour cream in two additions and the flour mixture in three additions, blending well after each.
Using a 1/4 cup ice cream scoop or measure cup, scoop mounds of the dough onto the baking sheets, spacing them 3 inches apart. Moisten your palm to prevent sticking and pat the mounds into 2 1/2-inch disks. Bake the cookies, one sheet at a time, for 15 to 17 minutes, until just beginning to turn light golden brown. Let the cookies cool on the baking sheet for 2 minutes, then transfer them to a wire rack to cool completely.
Make the white glaze
In a small bowl, whisk together the confectioners' sugar, heavy cream, and vanilla extract until smooth. Cover the bowl and set aside.
Make the chocolate glaze
Place the chocolate, heavy cream, and corn syrup in the top of a double boiler over barely simmering water. Heat, stirring occasionally, until the chocolate is melted and the glaze is smooth. Let the glaze cool until tepid, about 10 minutes.
Ice the cookies
Using a small offset metal spatula, spread the white glaze on half of each cookie. Let the glaze set for 10 minutes.
Using the spatula, spread the chocolate glaze on the other half of each cookie. Let the cookies set at room temperature for at least 45 minutes. Makes about 14 large cookies.
Source: The Good Cookie by Tish Boyle, 2002