Combine flour, cornstarch, ground almonds and salt in a bowl and stir with a fork until uniform. Reserve.
Stir butter and sugar in a mixing bowl and beat with an electric mixer until well combined. Beat in egg yolk, vanilla and almond extract. Add reserved flour mixture one-third at a time and beat until incorporated.
Transfer mixture to a large piping bag with a ¾-in. star tip and pipe 1½-in.-diameter “kisses” onto a parchment-lined baking sheet. Press a sliced almond into top of each cookie. Bake for 13 to 15 min or until lightly golden. Let cool.
Nutrition (per serving)
Make-ahead tip: Bake and freeze cookies in a sealed plastic bag for up to 2 months.