Cream the butter by beating it; add the eggs and continue beating. Add the sweetened condensed milk and the cornstarch along with the rice flour, previously sifted; beat until fully combined. Refrigerate for 45 minutes.

Form small balls with the dough and place them on a greased baking sheet, leaving space between them; sprinkle with a little brown sugar and refrigerate again for 20 minutes. Bake for 15 to 20 minutes.