Cook the raspberries in a medium saucepan over medium heat, stirring frequently until the raspberries are broken down into a sauce. Pour the sauce through a fine mesh strainer to remove the seeds, and then pour the raspberry sauce back in the pot. Simmer until the sauce reduces to a 1/4 cup. It will be a very rich red and concentrated sauce. Set aside to cool.
With a mixer, cream the butter with a paddle attachment on medium high speed about 2 minutes, until light and fluffy. Add 2 cups of powdered sugar, the 1/4 cup of cooled raspberry sauce, lemon juice, and salt. Mix until smooth. Add another 1-1/2 cups of powdered sugar and beat until smooth. If the frosting is too stiff, add a little milk (a tablespoon at a time) to thin it out.