Preheat oven 325F. Line baking sheet with silicone baking mat or parchment paper.
In a small mixing bowl, whisk egg white, salt, vanilla extract and sugar until foamy. Whisk until sugar is completely dissolved. Whisk in flour until no lumps. Then, gently fold in almond slices and butter.
Spoon 1 to teaspoon of batter onto prepared baking sheet and spread batter into a 2 to 3-inch circle. Make sure the almonds flakes are not overlapping each other. Spoon the rest of the batter on baking sheet with each cookie at least 1-inch apart. Might need to bake cookies in batches.
Bake 9 to 12 minutes depending on their size, rotate baking sheet once during baking. Bake until slightly golden brown. Remove from baking sheet and cool completely on wire rack. Cookies will crisp up once completely cool.