In a medium bowl, stir together flour, sugar, and salt. Using a pastry blender or 2 forks, cut in cold butter until it resembles coarse crumbs.
In a small bowl, whisk egg; add to flour mixture, stirring until clumps form. Divide and shape streusel into 15 balls.
On a lightly floured surface, press each ball into a 2¼-inch patty. Place on a large plate, separating layers with plastic wrap or parchment paper to keep from sticking together. Cover with plastic wrap, and refrigerate until ready to use, within 24 hours.
Recipe by TeaTime Magazine at https://teatimemagazine.com/conchas-mexican-sweet-rolls/