Combine blueberries, sugar, lemon juice and salt in a small or medium-sized saucepan and stir well.

Place on stovetop over medium-low heat and stir frequently until the berries are softened and release their juices. Smash the berries with your spatula (or with a potato masher) as they soften.

Bring mixture to a gentle simmer or slow bubble (don’t bring to a full rolling boil, reduce the stovetop heat if needed) and cook until the mixture is thickened and the spoon leaves trails in the sauce.

Pour through a fine mesh strainer into a heatproof container. Squeeze as much of the sauce through the strainer as possible and discard the remnants. You should have approximately ¼ cup (65g) of sauce (if you have less that’s fine, but if you have more it may not be reduced enough).

Allow to cool completely before proceeding with the recipe.

Place butter in a large mixing bowl and use an electric mixer to beat until light and smooth.

Gradually add powdered sugar to the butter, stirring until completely combined.

Add the cooled blueberry sauce by stirring in 1 Tablespoon at a time until completely combined. You should only have about 4 Tablespoons (66g) of sauce or less, if you have more than this your sauce may be thinner so check the consistency at 4 Tablespoons before proceeding. Adding too much sauce could cause the frosting to break.