Wash the peaches thoroughly under cold water. Peel the peaches using a sharp knife or blanching method, then cut them in half, remove the pits, and finely chop the flesh.

Measure out 4 cups of chopped peaches and transfer them to a large, heavy-bottomed saucepan.

Place the saucepan with the chopped peaches over medium heat.

Add the granulated sugar, lemon juice, and vanilla extract (if using). Stir well to combine.

Bring the peach mixture to a gentle boil, stirring frequently to prevent sticking. Allow the mixture to boil for 18-20 minutes, or until the peaches have softened and the mixture has thickened.

To test if the jam is ready, place a small amount of the mixture on a chilled plate and tilt the plate. If the jam doesn't run a lot, it's ready. If it's too runny, continue cooking for a few more minutes and test again.

If you prefer a thicker jam, you can add pectin to the mixture during the cooking process. Add the pectin over the boiling jam and stir well to dissolve. Continue cooking for an additional 1-2 minutes, then remove the saucepan from the heat.

Once the jam reaches the desired consistency, remove the saucepan from the heat and let the jam cool slightly.