Preheat oven to 300°F. Put almond paste, sugar, and salt in the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until crumbly, about 3 minutes. Add egg white and vanilla. Mix until smooth and thickened, about 3 minutes.
Drop batter by tablespoons onto baking sheets lined with parchment paper, spacing 2 inches apart. Place two almond slices on each mound of dough. Bake until cookies are golden brown, 20 to 25 minutes, rotating sheets halfway through.
Let cool completely on a wire rack. Just before serving, lightly dust cookies with confectioners’ sugar. Cookies can be stored between layers of parchment in airtight containers at room temperature up to 3 days.
