In a pan, toast flour on moderate heat for about 15 minutes, or until light brown, stirring constantly to avoid burning.
Remove the pan and transfer the mixture into a big bowl.
Add the powdered milk, and toss around for another 3-4 minutes.
Add sugar and melted butter. Mix well.
Fill the polvoron mold with the mixture, press it hard by using a spoon, then release it. If it is still too loose, add more butter or olive oil. Make sure that you can pick up the polvoron without it crumbling straight away.
Place the polvoron in an airtight container, then chill in the fridge until firm.
Carefully wrap the polvoron individually in Japanese paper or cellophane.
Keep the polvoron refrigerated until you want to eat them. You can store them in the fridge for about a week, or you can freeze them.