In the heatproof bowl of a stand mixer, whisk together egg whites, sugar, salt, and cream of tartar by hand. Place bowl over a saucepan of simmering water. Cook, whisking constantly, until sugar dissolves and an instant-read thermometer registers 160°F (71°C).

Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until stiff peaks form and meringue is just warm to the touch, about 10 minutes. Add butter, about2 tablespoons (28 grams) at a time, beating until combined after each addition. (If buttercream breaks or looks loose, beat for 2 to 3 minutes, and the emulsion will come back together.) Beat in vanilla.

With mixer on medium speed, add Roasted Strawberry Jam, 1 tablespoon (19 grams) at a time, beating until well combined. Beat in food coloring (if using). Use immediately, or refrigerate in an airtight container for up to 3 days. If refrigerating, let come to room temperature and re-whip before using.