Pistachio-Orange Cookies

Makes about 3 dozen sandwich cookies.
For cookies
2 cups flour
1 teaspoon salt
2 teaspoons cream of tartar
1 teaspoon baking soda
1 cup (2 sticks) butter, at room temperature
½ cup granulated sugar, plus extra for rolling dough
½ cup firmly packed brown sugar
1 egg
1 teaspoon vanilla extract
For filling
1 cup raw shelled pistachios, divided
½ cup (1 stick) butter, at room temperature
Freshly grated zest of 1 orange, finely chopped
1 teaspoon orange extract
1 teaspoon vanilla extract
3 cups powdered sugar
2 tablespoons milk (or use 1 tablespoon milk and 1 tablespoon freshly squeezed orange juice)
Directions
To prepare cookies:
Preheat oven to 350 degrees and line baking sheets with parchment paper.
In a large bowl, whisk together flour, salt, cream of tartar and baking soda, and reserve.
In a bowl of an electric mixer on medium-high speed, beat butter, granulated sugar and brown sugar until light and fluffy, about 2 minutes. Add egg and vanilla extract and beat until thoroughly combined. Reduce speed to low and add flour mixture, in thirds, mixing until just incorporated.
Form dough into ½-inch balls (about the size of a chestnut). Roll dough balls in granulated sugar and place 2 inches apart on prepared baking sheets. Bake until cookies are set but not browned, about 11 minutes (cookies will puff up in oven, but flatten). Remove from oven and cool 2 minutes before transferring cookies to a wire rack to cool completely.
To prepare filling:
In food processor fitted with a metal blade, pulse ½ cup pistachios until very fine (the nuts should almost clump together in a paste between your fingers).
In a bowl of an electric mixer on medium-high speed, beat butter with orange zest until light and fluffy. Add orange extract and vanilla extract and mix until thoroughly combined.
Reduce speed to low and add powdered sugar, in thirds, alternating with milk (or milk and orange juice), beginning and ending with powdered sugar, and beat until smooth (you may need another tablespoon or so of liquid to reach desired consistency). Fold in pistachios, mixing until thoroughly incorporated.
To assemble cookies:
Finely chop remaining ½ cup pistachios and place in a shallow dish. Spread a generous dollop of filling on bottom of a cookie. Top with another cookie, bottom-side up as if making a sandwich, and gently press cookies together until filling is just at edge of cookies. Roll filled edge in chopped pistachios. Repeat until all cookies form sandwiches.
2010 Winner: Scott Rohr of Minneapolis, Minnesota