Rinse the blueberries and remove any stems, then measure out all ingredients. In a heavy-bottomed pot, combine the blueberries, granulated sugar, lemon zest, lemon juice, and salt. The lemon juice provides natural pectin which helps the jam set properly, while the zest adds bright citrus flavor that complements the blueberries beautifully.

Place the pot over medium-high heat and stir occasionally until the mixture comes to a rolling boil. Once boiling, reduce the heat to medium and simmer for 7-10 minutes, stirring frequently to prevent sticking and ensure even cooking. I like to crush some of the berries with the back of my spoon as it cooks—this releases more juice and pectin from the fruit, which helps thicken the jam naturally while keeping some whole berries for texture. Continue cooking until the mixture reaches 220°F on a candy thermometer (the gel point for jam), or until a small spoonful placed on a cold plate wrinkles slightly when pushed with your finger.

Remove the pot from heat and stir in the vanilla extract, which adds a subtle depth that enhances the blueberry flavor. Let the jam cool for about 5 minutes—this brief rest allows the mixture to thicken slightly and helps prevent splattering when you transfer it. Pour the warm jam into sterilized jars, leaving about 1/4 inch headspace. I find that letting jam cool slightly before jarring gives you better control and reduces the risk of burns.