
Add the raspberries and lemon juice to a saucepan over medium heat. After the mixture heats up for a few minutes, mash the raspberries then allow it to simmer for about 10 minutes.
Strain the mixture into a bowl through a fine mesh sieve, and discard any solids. Cover the raspberry mixture and chill until cold, about 2 hours.
Cream the room temperature butter for a few minutes on medium speed with an electric or stand mixer.
Gradually beat in the confectioner’s sugar while mixing on low speed, and add the raspberry reduction a tablespoon at a time until the desired color and consistency are reached. (I usually end up using 1/4 cup.) Use the frosting immediately.