Combine flour, salt and baking soda in bowl. Beat butter and sugar in large bowl until creamy. Beat in egg, lemon extract and vanilla until fluffy. Stir in flour mixture and ground almonds. Divide dough in half; shape into disks. Wrap in plastic wrap. Refrigerate 1 hour.
Heat oven to 350 degrees F. On lightly floured surface with floured rolling pin, roll half of dough 1/4 inch thick. With floured 3-1/4-inch fluted round cookie cutter, cut out dough. Place, 1 inch apart, on ungreased baking sheets. With floured 1-inch round cookie cutter, cut out centers from half the cookies. With other half of dough and cutout centers and trimmings, repeat rolling and cutting out more cookies.
Bake in 350 degree F oven 12 to 15 minutes or until lightly browned. Transfer cookies to wire racks to cool. Store in airtight container at room temperature or in freezer.
Beat together warm water and powdered egg whites in medium-size bowl until frothy. Gradually add confectioners sugar and beat for 7 to 10 minutes or until shiny, smooth and good spreading consistency. Divide icing into quarters. Leave one quarter white, and tint other 3 batches desired colors for flowers. Place white icing in pastry bag fitted with small round tip.
Spread tinted icing over cookies with cut-out centers. Set aside to dry. Pipe petal design with white icing over tinted icing on top of each cookie. Let dry completely. Spread 1 tablespoon lemon curd over each cookie without cut-out center; top with iced cookie, pressing lightly together to form sandwich. Place 1/4 teaspoon of the nonpareils in curd in center cutout of each cookie. Makes about 16 cookies.