First, read this blog post for a detailed step-by-step tutorial, this will help you in the long run I promise! Whisk egg whites and sugar in the the clean bowl of a stand mixer (or another heat-proof bowl) and set over a saucepan with 2 inches of simmering water on medium heat.
Cook egg mixture, stirring occasionally, 5 to 7 minutes or until mixture registers 160°F on an instant-read thermometer and sugar granules are completely dissolved. To test if the sugar is completely dissolved, quickly dip a finger into the mixture and rub between your fingers. It should be completely smooth.
Transfer bowl to a stand mixer fitted with the whisk attachment and beat on high speed until stiff peaks form, about 7 to 10 minutes. If you hit 10 minutes and still don't have stiff peaks, place the entire bowl in the refrigerator for exactly 10 minutes. Then, remove the bowl and continue beating until stiff peaks form.
Once you reach stiff peaks, if the bowl and meringue are still warm to the touch, refrigerate for 10 minutes exactly and proceed to the next step.
With the mixer running on medium-low speed, beat in half of the butter (3/4 cup or 1 1/2 sticks), 1 Tbsp at a time, waiting until 1 Tbsp is completely incorporated before adding the next.
Switch to the paddle attachment and continue beating in remaining butter on medium-low speed, still adding it 1 Tbsp at a time until buttercream is smooth, fluffy, and holds its shape.
Beat in strawberry jam, vanilla, salt, and food coloring on low speed until smooth. Transfer buttercream to a piping bag fitted with desired tip and pipe onto cupcakes.
If the buttercream is curdled: Sadly, the SMBC may have separated and the result is small flecks of butter that look slightly chunky and thick. The butter may have been colder than the meringue when you began beating it, so here's a super quick fix: remember our saucepan of simmering water in the beginning? Bring that water back to a simmer and place the bowl over the water. Let it sit, without mixing, for 1 to 2 minutes or until the edges of the meringue warm up and become liquid. Then, transfer the bowl back to the mixer and beat on low speed for 30 seconds, then medium-high speed until it becomes smooth (usually about 1 to 2 minutes).
If the buttercream is soupy: Unfortunately, your butter may have become too soft in the meringue when you added it, resulting in a warm SMBC. Simply take a bag of frozen peas (or whatever you have in your freezer) and hold it on the sides of the bowl while it's mixing on low speed. After holding it here for a few minutes the mixture should cool off and begin to thicken. BUT, if it's still not thickening, place the entire bowl in the refrigerator in 20 minute spurts. Then, place the bowl back on the mixer and beat on high speed until it thickens up, about 1 to 2 minutes.
