Add the flour to a skillet and toast over medium heat, stirring regularly until lightly browned and aromatic. Remove from the heat and allow to cool.
In a bowl, whisk together the toasted flour, powdered milk, and sugar. Add the popped rice, then the two butters and continue to mix until well combined.
On a baking sheet, place a circular cookie cutter. Fill with the polvorón powder and use the bottom of a glass to tamp it down compactly, forming a puck. Repeat until you’ve used all the powder, then place the baking sheet in the freezer until firm, for about 30 minutes.
Wrap individually in cellophane or wax paper.