Combine raspberries, sugar, lemon juice and salt in a small saucepan over medium heat.

Cook, stirring frequently, until berries release their juices and berries are softened. Smoosh the berries with your spoon or spatula as you stir until berries are completely broken up.

Reduce heat to a simmer and continue to cook, stirring constantly until mixture is thickened – the spoon or spatula should leave trails in the sauce.

Set a fine mesh strainer over a heatproof bowl and pour raspberry sauce through the strainer. Use a spatula to stir the mixture to encourage as much juice through the strainer as possible.

Discard the seeds caught in the strainer. I recommend measuring the sauce, you should have 3 ½-4 Tablespoons after straining. If you have significantly more than this you may not have reduced your raspberry sauce enough and should return it to the saucepan to thicken a bit longer.

Allow sauce to cool completely before proceeding.

Once sauce has cooled, place butter in a large bowl and use an electric mixer to beat until smooth and creamy.

Gradually add powdered sugar until completely combined.

Add raspberry sauce gradually, about 1 Tablespoon at a time and stirring until completely combined.

Use frosting as desired.