For the cookies:
2 cups all-purpose flour
1 cup cornstarch
1/2 tsp salt
3 sticks unsalted butter, room temperature
3/4 cup powdered sugar
1 Tbl plus 1 ½ tsp finely grated lemon zest (from about 3 lemons)
1 1/2 tsp vanilla
½ cup plus ¼ cup granulated sugar, divided, for dusting
2 Tbl yellow sugar, for dusting
For the filling:
1 cup powdered sugar
1 Tbl finely grated lemon zest (from about 2 lemons)
1 Tbl fresh lemon juice
2 Tbl light corn syrup
½ stick unsalted butter, softened
To make the cookies: Preheat the oven to 350°F and arrange a rack in the middle position. Line 2 large baking sheets with parchment paper.
In a medium-sized mixing bowl, whisk together the flour, cornstarch and salt. Set aside.
Combine the butter and powdered sugar in the bowl of an electric stand mixer fitted with the paddle attachment. Beat on medium-high speed until light and fluffy, about 3 minutes. Add the lemon zest and vanilla and beat to combine. With the mixer running at low speed, gradually add the flour mixture. Continue to mix just until a soft dough forms.
Add the half cup granulated sugar to a small bowl. In a separate small bowl, combine the remaining ¼ cup granulated sugar with the 2 Tbl yellow sugar and stir to combine.
One by one, roll scant half tablespoons of dough into balls, then drop into one or the other bowls of sugar, turning to coat completely. Drop half into the plain white sugar and half into the white-yellow sugar mixture. (You could simply do one or the other, but for some reason, I really like the combination.) Transfer the sugar coated balls to the parchment-lined baking sheet, spacing the balls about ¾-inch apart, until the baking sheet is filled.
Refrigerate for 30 minutes. (This will help preserve their rounded shape.)
Bake, one baking sheet at a time, until the tops are lightly cracked but the cookies are still pale, about 15 minutes. (The bottoms should be pale golden.) Transfer the cookies by their parchment to a wire rack. Cool completely to room temperature. The un-frosted cookies can be kept at room temperature in an air-tight container.
To make the filling: Combine the powdered sugar, lemon zest, lemon juice, corn syrup and butter in the (cleaned and dried) bowl of an electric stand mixer fitted with a paddle attachment. Beat at medium speed until well combined, about 1 to 2 minutes. Transfer to a heavy-duty plastic bag. Snip off a corner to form a small hole.
To assemble the cookies: Flip over half of the cookies to expose their flat bottom sides. Pipe about ½ teaspoon of filling onto the center of each cookie bottom. Sandwich with the remaining cookies, matching like colors, white with white and yellow with yellow, pressing gently to secure them. Keep the finished cookies refrigerated in an air-tight container.