Line 2-3 large baking sheets with parchment paper or silicone baking mats, and set aside.
Melt the butter in a medium saucepan on medium heat. Once melted, add the brown sugar, almond flour, salt, and milk. Cook and whisk until sugar has dissolved and ingredients are completely combined, around 3 minutes.
Remove from heat and whisk in the vanilla extract. The mixture will be grainy and shiny. Allow the cookie dough to sit and thicken for about 5-10 minutes. Meanwhile, preheat the oven to 350°F. The mixture will thicken as it cools down.
Drop scant teaspoonfuls of the mixture 3-inches apart onto the prepared baking sheets. Bake for 6-8 minutes until golden brown around the edges. The edges and centers will sizzle and bubble as the cookies bake.
Allow the cookies to cool for 5 full minutes on the baking sheets before transferring to a rack to cool completely. Cookies dry and crisp up as they cool.
Once cool, dust with powdered sugar, or spread either melted chocolate, Nutella, or jam onto the bottom of one cooled cookie and sandwich with another.