First, bring your butter to room temperature. Here is how to soften butter. In the meantime, prepare a stand mixer with a paddle attachment. Alternatively, use your electric hand mixer.
Puree the raspberries and run it through a sieve to get rid off the seeds. Then, cook the mixture in a medium saucepan together with the granulated sugar and lemon juice over medium heat for 10-15 minutes or until the mixture is reduced to 60g / ⅓ cup. Then, let the mixture come to room temperature.
Cream the room-temperature butter at medium-high speed for 3 minutes until it becomes light in color and fluffy in texture.
Next, add the sifted powdered sugar and continue whipping for another 2 minutes until the mixture is pale and fluffy.
Then add the full fat cream cheese and continue whipping for 2 more minutes.
Now, very slowly, start adding the room temperature raspberry reduction one tablespoon at a time with the mixer running. You can stop whipping once the raspberry mixture is incorporated.
Transfer the frosting into a piping bag and chill it for 30 minutes before using.
The frosting can be stored refrigerated for 2-3 days.
Notes
The recipe above is enough to frost 12 raspberry cupcakes.To frost cakes, you will need approximately 2x for a 6-inch cake, 2.5x for a 7-inch cake, and 3x for an 8-inch cake (depending on how much frosting vs. sponge ratio you prefer on your cakes and whether you are using any other filling in between the layers).
Make sure you read my Expert Tips section above to maximize your success. A short recipe alone is not able to cover all the necessary details and science behind baking.
Measure your ingredients with a digital scale for accuracy.
Alternatively, instead of the raspberry reduction, use good quality seedless raspberry jam.
You may further enhance this frosting with a small amount of freeze dried raspberries (¼ cup). However, it is not required, the raspberry reduction alone will provide a lovely color and raspberry flavor.
The cream cheese needs to be high in fat. Using the low-fat version may result in a thinner frosting due to the excess water content.
Make sure to let the raspberry reduction come to room temperature before adding to the cream cheese frosting.
Adding the fruit reduction to the frosting needs to happen very slowly step-by-step or the frosting might break.
Because softened butter is being used, the frosting may still be on the softer side right after it's whipped. If this is the case, chill it for 30 minutes before piping. However, if it's chilled for too long, the butter will solidify, and you might find it difficult to pipe. If this happens, just need to let it soften a little before using it.