Prepare a stand mixer bowl and whisk attachment by wiping down with some white vinegar on a paper towel. This will remove any oils that may be left on the equipment that can ruin the meringue.
Fill a medium saucepan with a couple of inches of water. Place over medium-low heat and bring to a gentle simmer.
Hand whisk egg whites and sugar in a heat-proof bowl that can sit on top of the saucepan without touching the water (like a bain marie). Heat the egg mixture over the simmering water, stirring constantly, until the mixture reaches 160°F (71°C) on an instant-read thermometer.
Once the egg and sugar mixture has reached temperature, transfer to the stand mixer bowl. Whip on high speed until the egg whites reach stiff peaks and the mixing bowl is no longer warm to the touch. To test the meringue stiffness, pull the whisk out. A stiff peak should stand straight up (no curl at the tip). You'll also know it's ready because you can fully flip the bowl over and nothing moves or falls out. Make sure not to overbeat the meringue at this stage.
With the mixer running on medium speed, add in one cube of butter at a time, allowing the butter to fully incorporate before adding in another cube. Once all the butter has been added, the buttercream should be light and smooth. Beat in strawberry jam, vanilla extract, and salt. Scrape down the sides of the bowl to ensure all the ingredients get incorporated and allow the mixture to beat for an additional two minutes on low speed. If at any point your buttercream becomes runny or curdled, see troubleshooting notes below.
Deflate the buttercream by hand using a rubber spatula. Press the buttercream around the side of the mixer bowl. Continue until most of the air bubbles have disappeared. This helps the buttercream pipe smoothly.
Prepare a piping bag with a round tip (Wilton 2A). Push part of the side of the piping bag into the piping tip to ensure nothing leaks out. Set the bag inside a tall glass, and pull the remaining top of the piping bag out over the outside of the glass to make filling the piping bag easy and clean. Pour the buttercream into the piping bag.