A sprinkling of sugar gives flowers baked onto scalloped sugar cookies a lovely, subtle iridescence.
Line two baking sheets with parchment paper. In a large bowl, sift together flour, salt, and baking powder. Set aside.
In bowl of electric mixer fitted with paddle, cream butter on medium speed. Gradually add granulated sugar; beat until fluffy. Add eggs gradually; beat until fluffy. On low speed, add vanilla and reserved dry ingredients; mix until combined. Divide dough into two balls; wrap in plastic wrap. Flatten into disks. Refrigerate 30 minutes.
Heat oven to 325 degrees. In a small bowl, whisk egg white with 1 tablespoon water until frothy. On a lightly floured surface, roll out one ball of dough to a 1/4-inch thickness.
Using 1 1/2- or 2 1/2-inch-round fluted cookie cutters, cut out shapes. Place on baking sheets, spacing 2 inches apart. Brush tops with egg wash. Place a pansy on top of each cookie; affix petals with more egg wash. Sprinkle sanding sugar over pansies. Bake until lightly golden, 8 to 10 minutes. Transfer to wire rack to cool. Repeat with remaining dough. Store in an airtight container up to 1 week.