labor intense, but very pretty !

Cookies:
1⅓ cups all-purpose flour
⅔ cup rice flour
¼ teaspoon salt
2 sticks unsalted butter,softened
½ cup confectioners sugar
1 tablespoon grated lemon zest
1 teaspoon vanilla
White or colored sanding sugars
Filling:
1 cup confectioners sugar
1 tablespoon grated lemon zest
1 tablespoon fresh lemon juice
2 tablespoons light corn syrup
½ stick unsalted butter, softened
1 heavy-duty re-sealable bag

1 ⅓ cups all-purpose flour 2 sticks unsalted butter,softened 1 tablespoon grated lemon zest 1 teaspoon vanilla 1 cup confectioners sugar 1 tablespoon grated lemon zest 1 tablespoon fresh lemon juice 2 tablespoons light corn syrup 1 heavy-duty re-sealable bag

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 tsp 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 cookies:

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 tsp filling on flat side of each. Sandwich with remaining cookies, pressing gently.