Transfer cajeta to a pastry bag fitted with a plain 1/4-inch tip. Flip half the cookies bottom side up and pipe a generous dollop of cajeta into the very center, up to 1/2 ounce (4 teaspoons; 14g), or less if you prefer. Alternatively, this can be done with a spoon. Top with remaining wafers and press gently to secure. Dust with powdered sugar and serve. In an airtight container, alfajores will keep 24 hours at room temperature or up to 1 week in the fridge.
Special equipment
Food processor, rolling pin, 2-inch cookie cutter, offset spatula, aluminum half-sheet pan
Notes
Cajeta can be prepared and refrigerated up to a month in advance. Not only does that make alfajores a breeze to bake and assemble, it ensures that the cajeta has a chance to thicken and cool.
Tapioca starch breaks down at relatively low temperatures, eliminating the starchy aftertaste associated with traditional cornstarch-based recipes. Powdered sugar dissolves more readily in a low-moisture dough, leaving no trace of grit.
