Gather ingredients: 2 cans (14 oz each) sweetened condensed milk (or 2 cups whole milk + 1 cup sugar + 1/4 cup powdered milk)
Option A (cook-and-reduce method):
Pour 2 cups whole milk into a heavy-bottom saucepan
Add 1 cup sugar
Add 1/4 cup powdered milk (optional for thicker texture)
Whisk until sugar dissolves
Simmer on low to medium-low, stirring frequently, until reduced to about 3/4 to 1 cup (thick and syrupy)
Stir in 1/2 tsp vanilla (optional)
Cool to thicken further
Option B (water-bath method using unopened cans):
Remove labels from unopened cans
Place cans in a large pot and cover with water by at least 1–2 inches
Bring to a boil, then reduce to a steady simmer
Simmer 2 1/2 to 3 hours for thick condensed milk
Keep water level covered throughout cooking
Turn off heat and let cans cool completely in the pot
Store:
Refrigerate in an airtight container up to 1 week
Use:
Stir well before using; warm gently if needed for consistency