Preheat oven to 400°F. Line a small baking sheet with aluminum foil. Spread almonds on prepared baking sheet in an even layer in an oval shape, with all almond pieces touching each other to prevent burning. Bake in preheated oven until lightly browned, 5 to 8 minutes. Let cool 10 minutes. Reduce oven temperature to 350°F.

While almonds are cooling, beat butter and granulated sugar in a stand mixer fitted with the paddle attachment on medium speed until light and fluffy, about 3 minutes. Alternatively, beat with a hand mixer on medium-high speed. Scrape down sides and bottom of bowl using a rubber spatula. Add vanilla; beat on low speed until combined, about 10 seconds.

Stir together flour and baking powder in a medium bowl. With stand mixer running on low speed or a hand mixer on medium speed, gradually add flour mixture to butter mixture, about 1 tablespoon at a time, beating just until incorporated after each addition, 2 to 3 minutes. Add toasted almonds; beat on low speed until evenly distributed, about 1 minute.

Line a large baking sheet with parchment paper. Divide dough into 18 to 20 balls (about 2 tablespoons each). Transfer 1 dough ball to a clean work surface, and roll into a 4-inch-long, about 1/2-inch-thick log. Shape into a wide crescent, and gently pinch ends. Transfer to prepared baking sheet with crescent arc pointing to short edge of baking sheet to fit all cookies on sheet. Repeat process with remaining dough balls, spacing about 1 inch apart.

Bake at 350°F until cookies look dry and are very light golden brown on tops and bottoms, 20 to 22 minutes. Transfer baking sheet to a wire rack; let cool completely on baking sheet, about 30 minutes.

Place powdered sugar in a small, shallow bowl. Working with 1 cookie at a time, gently coat in powdered sugar. Alternatively, sift powdered sugar over tops of cookies.