Chocolate Dipped Lime Cookies

1. Preheat the oven to 350 degrees F. Butter two cookie sheets. Sift the flour, cornstarch, and baking powder into a medium bowl. Beat the butter and confectioners' sugar in a large bowl with an electric mixer at high speed until creamy. Add the egg yolk, lime zest and lime juice, and vanilla, beating until just blended. Mix in the dry ingredients. Beat the egg white and salt in a large bowl with an electric mixer at high speed until stiff peaks form. Use a large rubber spatula to fold the beaten white into the lime mixture.
2. Fit a pastry bag with a 1/2-inch plain tip. Fill the pastry bag, twist the opening tightly closed, and squeeze out 2-inch lines, spacing 1 inch apart on the prepared cookie sheets. Bake for 8-10 minutes, or until just golden, rotating the sheets halfway through for even baking.
3. Cool on the sheets until the cookies firm slightly. Transfer to racks to cool. Melt the chocolate in a double boiler over barely simmering water. Dip the cookies halfway in the chocolate, sprinkle with the pistachios, and let stand on the parchment paper for 30 minutes until set.

Yield: 30 servings