1. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy, about 4 minutes. Add eggs and vanilla; beat until thoroughly combined. Beat in 2 cups flour, baking powder, and salt.

2. Divide dough in half; transfer one half to another bowl. To the remaining half in mixer bowl, add 1 cup flour and 1 teaspoon water; beat until combined. Transfer to a piece of plastic wrap. Return other dough half to mixer bowl; add cocoa powder and remaining 1/2 cup flour. Beat until combined. Transfer to another piece of plastic wrap. Flatten both halves into disks, and wrap well. Chill at least 30 minutes or overnight.

3. Turn out plain dough onto a lightly floured surface; divide in half. Roll out one half about 1/8 inch thick. Using a paring knife or cookie cutter, cut dough into 1 1/2-inch squares; arrange on a parchment-lined baking sheet. Roll out other dough half; cut just enough additional squares to total 52. Place on another parchment-lined baking sheet; chill until firm, at least 20 minutes.

4. Meanwhile, using a 1 1/4-inch cookie cutter, cut out rounds from remaining plain dough. Reroll scraps, if desired. Place on a parchment-lined baking sheet, about 3/4 inch apart; chill until firm.

5. Remove chocolate dough from the refrigerator, and divide in half. On a lightly floured surface, roll out one half about 1/8 inch thick. Brush off excess flour with a dry pastry brush. Using 1-inch letter-shaped cookie cutters (dip in flour before each cut), cut out 2 of each letter (for a total of 52 letters), and using a thin spatula, transfer to a parchment-lined baking sheet. If dough stays in cutter, gently tap the side of the cutter against the counter or your hand to remove the letter.

6. Using the same 1 1/4-inch cookie cutter, cut out additional rounds from remaining chocolate dough. Reroll scraps, if desired. Place on baking sheet with plain rounds; chill until firm.

7. Preheat oven to 350 degrees. Remove squares and letters from refrigerator. Using a pastry brush or your fingertip, dampen the top of each square with water; center a letter on top of each square. Transfer to baking sheets, about 3/4 inch apart. Bake until just starting to brown, 9 to 10 minutes, rotating baking sheets halfway through cooking time. Transfer cookies to wire racks to cool completely.

8. Remove rounds from refrigerator, and bake until just starting to brown and turn crisp, 9 to 10 minutes. Transfer cookies to wire racks to cool. Cookies can be stored in airtight containers at room temperature up to 5 days.