Combine the sugar and water in a medium saucepan. Heat over medium heat, stirring until the sugar dissolves.

Allow the syrup to boil without stirring until it reaches 240°F (soft-ball stage). Use your instant-read thermometer for accuracy.

While the syrup cooks, place the egg whites in the bowl of your stand mixer. Beat on medium speed until soft peaks form.

Once the syrup reaches 240°F, carefully pour it in a thin stream into the whipped egg whites. Keep the mixer running on medium-high speed.

Continue beating until the meringue cools to room temperature and forms stiff peaks. This can take about 5-10 minutes.

With the mixer on low speed, add the softened butter a few tablespoons at a time. Allow each addition to fully incorporate before adding more.

Once all the butter is added, mix in the vanilla extract. Taste and adjust flavoring as desired.

Your Italian Meringue Buttercream is now ready! Use it to frost cakes or store in an airtight container for later use.