Lemon Lime Refrigerator Cookies

Line rimless baking sheets with parchment paper or grease; set aside.

In large bowl, beat butter with sugar until fluffy. Beat in egg, then lemon and lime rinds. In separate bowl, whisk together flour, baking powder and salt ; stir into butter mixture, one-third at a time; stir in white chocolate (if using).

In small bowl, beat egg white until foamy. Sprinkle almonds on waxed paper. Set both aside.

Divide dough into quarters. Place, 1 at a time, on large piece of waxed paper. Using paper as guide, roll and press firmly into 6-inch (15 cm) long log. Remove paper. Brush with egg white; roll in about one-quarter of the almonds. Wrap in plastic wrap, twisting ends to seal. Refrigerate until firm, about 3 hours, rerolling as logs harden to keep round shape. (Make-ahead: Refrigerate logs for up to 3 days or freeze in airtight container for up to 3 weeks. Let stand at room temperature for 20 minutes.)

With serrated knife, cut into generous 1/4-inch (5 mm) thick slices; place, about 2 inches (5 cm) apart, on prepared pans. Bake in top and bottom thirds of 375°F (190°C) oven, rotating and switching pans halfway through, until bottoms are light golden, about 10 minutes.

Glaze: Meanwhile, in bowl, mix icing sugar, cream, butter and lemon and lime rinds until smooth; spread over hot cookies. Transfer to racks to let cool. (Make-ahead: Layer between waxed paper in airtight container; store at room temperature for up to 5 days or freeze for up to 2 weeks.)