Put the raspberries in a small sauce pan and cook over medium heat stirring with a spoon until they break down into a sauce. Pour the sauce through a fine mesh strainer to take out the seeds and then put the seedless sauce back in the pot and simmer until it reduces to a 1/4 cup of very concentrated raspberry sauce. Set aside to cool.
When the raspberry flavoring is room temperature, cream the butter in the bowl of a standing mixer using a paddle attachment. Then add about 2 cups of powdered sugar, the raspberry sauce, 1/2 teaspoon of fresh lemon juice and a pinch of sea salt. Process until smooth. Add another 1 1/2 cups of powdered sugar and continue to mix until it is spreading consistency adding milk if needed to make it less stiff.
I adapted my frosting to make it vegan because my daughter is allergic to dairy. So I used expeller pressed coconut oil in place of the butter. I had to melt it slightly to remove the lumps as they wouldn't creamt together. It still came together very well. If the temperature was above 75 or so, the frosting might melt if using coconut oil. I also used coconut milk to thin in and it left tiny white lumps due to the acidity of the lemon juice. It would have worked better to use rice milk or even just water, probably.