Preheat oven to 350°F (175°C). Line a baking sheet with parchment.
Spread blanched almonds on a baking sheet and toast for 6–10 minutes, shaking once, until lightly golden. Cool completely.
Pulse toasted almonds in a blender/food processor until finely ground (stop before it turns pasty).
In a bowl, whisk flour, baking powder, and salt.
In a large bowl, beat softened butter and sugar until pale and fluffy.
Beat in the egg and vanilla. Mix in the ground almonds.
Add the dry ingredients and mix just until a soft dough forms.
Scoop and roll dough into balls. Place on the lined baking sheet with space between.
Brush tops lightly with egg white and press on almond flakes/sliced almonds.
Bake 10–14 minutes until set and lightly golden on the bottom. Cool 5 minutes on the pan, then transfer to a rack.