In a large bowl, sift together flour, salt, and baking powder. Set aside.
In the bowl of an electric mixer fitted with a paddle attachment, cream butter and sugar until light and fluffy. Add the egg; beat until combined. Gradually add sifted ingredients, and mix on low speed until combined. Add vanilla. Shape dough into a disk, wrap in plastic, and chill until firm, about 30 minutes.
Heat oven to 325 degrees F.
On a lightly floured surface, roll dough to 1/4-inch thick. Using a 1 1/2-inch round cookie cutter, cut out about 4 dozen circles. Using a clean rubber stamp, imprint an initial into the center of each cookie. Transfer to ungreased baking sheets, and refrigerate until firm, about 15 minutes.
Bake until edges start to brown, about 8 to 10 minutes. Transfer cookies to a rack to cool.
In a small bowl, combine 3 tablespoons meringue powder with 2 tablespoons water until smooth. Decorate 1 cookie at a time; using a small paintbrush, lightly brush each imprinted initial with meringue. Sprinkle sanding sugar in the initial, and tap out excess over a small bowl. Using a toothpick or small dry paintbrush, gently remove the excess sugar in and around the letter. Brush the edge of each cookie with meringue. Roll the edge of each cookie in sanding sugar to coat. Transfer cookies to a cooling rack set over a sheet pan to dry completely, 1 to 2 hours. Cookies can be stored at room temperature in an airtight container up to 4 days.
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