Stir flour with baking powder and salt in a medium bowl. Add 2 tbsp instant-coffee granules to flour mixture. Set aside.
Beat butter with sugar in a large bowl, using an electric mixer on medium-high, until fluffy. Beat in egg and vanilla. Gradually beat in flour mixture, just until combined. Gather into a ball.
Position racks in top and bottom thirds of oven. Preheat to 350F. Line 2 baking sheets with parchment.
Roll into 1-in.-round balls and set on baking sheet. Make a deep indent in the centre of each ball. Bake in centre of oven until golden, 11 to 12 min. Let cookies cool on sheet for 1 min. Transfer to a rack to cool completely. Pipe or spoon a dulce de leche dollop in centre of each cookie.
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