In a small pot over medium heat, bring the raspberries to a simmer with ¼ cup water. Break them up after about 4-5 minutes over the heat. Reduce the heat to medium low/low and cook uncovered for about 15 minutes (to reduce the amount of liquid). Strain through a fine mesh sieve (use a spoon or spatula to push the liquid through until all you get left are seeds.) You’ll get about ? cup of puree. Set aside to let cool to room temperature. Meanwhile, to make the SMB, whip the egg whites and sugar together until they’re at stiff peak. Rub some between your fingers to make sure the sugar is dissolved (it should be by the time you get it to stiff peak). I think I whipped on high for about 10-15 minutes. Yes, I know generally you’re supposed to heat it up, but I was lazy. It worked though. Switch to a paddle attachment and beat, adding the room-temperature-ish butter, cube by cube, waiting until each cube is incorporated before you add another. Once you’ve added it all, add the raspberry purée and beat until incorporated. Transfer to a piping bag and pipe awaaay!