01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat butter and sugar with an electric mixer until light and fluffy, about 2 minutes. Add egg and vanilla; mix until combined.
04 - Gradually add flour mixture, mixing until just incorporated.
05 - Scoop dough by rounded tablespoons onto prepared baking sheets, spacing 2 inches apart. Flatten gently with your palm or the bottom of a glass.
06 - Bake for 10-12 minutes, until edges are just golden. Let cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
07 - Beat powdered sugar, butter, milk, vanilla, and salt until smooth and spreadable. Add extra milk if needed for creamy consistency.
08 - Once cookies are completely cool, spread a generous layer of frosting on each.
09 - Combine chocolate-hazelnut spread and milk in a small microwave-safe bowl. Microwave in 10-second bursts, stirring, until smooth and pourable.
10 - Drizzle mixture over frosted cookies using a spoon or piping bag. Allow drizzle to set before serving or storing.