Prepare the raspberry sauce first. Place the raspberries and granulated sugar into a small saucepan and bring the berries to a simmer over medium heat. Simmer for 4 to 5 minutes, until the berries start to fall apart and form a jam. Pour the jam into a fine mesh strainer arranged over a mixing bowl. Use the back of a spoon or a spatula to press the sauce through the strainer, leaving the seeds behind. Discard the seeds and place the sauce into the refrigerator to chill.
Next, prepare the buttercream. The butter needs to be just slightly softened, not melted. If the butter is too soft, return it to the refrigerate to chill. Place the softened butter into a mixer bowl and whisk on high speed for 7 to 10 minutes, until the butter is fluffy, creamy, and pale white in color. Make sure to scrap down the sides of the mixing bowl often to ensure all the butter is well-mixed.
Add the confectioner’s sugar next and start mixing on low speed first until the sugar is incorporated. Once all the lose sugar is incorporated, turn the mixer to high speed and whisk for 3 to 4 minutes, until the buttercream is light and fluffy. Scrap down the sides of the mixing bowl periodically to make sure everything is well mixed.
If desired, add a few drops of red food coloring at this time. For a sourer buttercream, add the citric acid. For even more raspberry flavor, add raspberry extract. Add the chilled raspberry sauce last! Add the sauce and mix for about 15 to 20 seconds, 30 seconds maximum, with the mixer. Do not overmix or this will cause the buttercream to separate! Finish mixing in the sauce by hand using a spatula.
Transfer the buttercream into a pastry bag or use to frost a cake or dessert. I used Ateco tip #847 in my video recipe for the piping demonstration.
This frosting can be refrigerated and used for later. Simply cover the frosting very well with plastic wrap and place into the refrigerate for up to 1 week. When ready to use, remove it from the fridge and allow the frosting to soften at room temperature for 1 to 2 hours. Once softened, mix well with a spatula until the frosting is uniform. If needed, re-whisk for a few minutes until the buttercream is fluffy again.