Preheat the oven to 350°F/175°C. Line 2 baking sheets with parchment paper.
Place almond slices on a baking sheet and bake at 350F for 3-5 minutes. Remove from the oven and let them cool completely.
In a small bowl, melt the butter in the microwave for about 15-20 seconds. Let it cool and set aside.
In a large mixing bowl, using a handheld mixer or a stand mixer, beat the egg white and sugar until the sugar dissolves. Add vanilla extract and melted butter to the mixture and whisk to combine. Add salt, flour and toasted almond slices to the mixture and gently fold to combine using a rubber spatula or a wooden spoon. The batter will be slightly diluted like condensed milk.
Pour the batter onto the prepared baking pan with parchment paper attached and spread evenly in the shape of a square/rectangle. It should be 1/8 - 1/10 inch thickness. This will help you easily cut into small thin squares. Bake for 10-15 minutes until the edges are golden brown. Remove from baking pan and while it’s still hot, cut into squares. Bring the cookies back to the oven and bake for extra 2-3 minutes until they are crispy.
If you prefer to make round cookies, you can spoon the batter on the baking pan with parchment paper attached and spread it out into a thin round shape with 1/8 - 1/10 inch thickness. Leave 2-3 inch space between the cookies. Bake them for 10-13 minutes until the edges are golden brown. By doing this, you may need to bake several batches. Before starting a new batch, let the baking sheet cool completely before adding batter onto it.
After removing from the oven, let the cookies cool completely on the rack. Tuiles are best eaten the same day it’s made. Store the cookies in an airtight container for up to a week.