COOKIE INGREDIENTS:

• 1/3 cups all-purpose flour
• 2/3 cup cornstartch
• 1/4 teaspoon salt
• 2 sticks [1/2 lbs] unsalted butter. softened
• 1/2 cup confectioners sugar
• 1 tablespoon grated lemon zest
• 1 teaspoon vanilla
• white & colored sanding sugars

FILLING INGREDIENTS:

• 1 cup confectioners sugar
• 1 tablespoon fresh lemon juice
• 2 tablespoons light corn syrup
• 1/2 (4 tablespoons) unsalted butter, softened

MAKE COOKIES:

• Preheat oven to 350°F with rack in middle. Line 2 large baking sheets with parchment paper.
• Whisk together flour, cornstarch, and salt.
• Beat together butter and confectioners' sugar with an electric mixer until pale and fluffy, then beat in zest and vanilla. At low speed, mix in flour mixture just until a soft dough forms.
• Put sanding sugars in different bowls. Roll a scant teaspoon of dough into a ball and drop into sugar, turning to coat. Reshape if necessary and transfer to a baking sheet. Repeat, spacing balls 3/4 inch apart, until baking sheet is filled.
• Bake until tops are slightly cracked but still pale (bottoms will be pale golden), 12 to 15 minutes. Transfer cookies on parchment to a rack to cool completely.
• Form and bake more cookies on second baking sheet.

MAKE FILLING AND SANDWICH:

• Beat together all filling ingredients in a large bowl with an electric mixer at medium speed until combined well. Transfer to sealable bag and snip off a corner.
• Turn over half of cookies and pipe about 1/2 teaspoon filling on flat side of each. Sandwich with remaining cookies, pressing gently.