Preheat oven to 400°. Line 3 rimmed baking sheets with parchment paper.
In a large bowl, beat together butter and confectioners’ sugar with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add egg, 1 tablespoon water, and food coloring, beating until well incorporated. Add vanilla extract, beating until combined.
In a medium bowl, whisk together flour, baking powder, and salt. With mixer at low speed, gradually add flour mixture to butter mixture, beating until a dough forms.
Turn out dough onto a heavily floured surface. Divide dough in half. Using a rolling pin, roll out one portion of dough to a ¼-inch thickness. Using a 2¼-inch tree-shaped cutter, cut 36 cookies from dough. Place on prepared baking sheets. Repeat with remaining dough portion. Lightly brush tops of cookies with water, and sprinkle with nonpareils.
Bake until cookies are firm but not browned, approximately 7 minutes. Let cool completely on wire racks.
Place chocolate-hazelnut spread in a piping bag fitted with a small round tip (Ateco #802). Pipe ½ teaspoon chocolate-hazelnut spread onto flat side of 36 cookies, following tree shape. Place remaining cookies, flat side down, on top of spread. Store at room temperature in an airtight container with layers separated by wax paper until needed, up to 4 days.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chocolate-filled-christmas-tree-cookies/