In a heavy-bottomed saucepan over medium heat, combine 3/4 cup sugar and 1/4 cup water and stir until the sugar dissolves.

Once the syrup comes to a boil, stop stirring and clip on a candy thermometer, cooking until it reaches 240°F (soft ball stage), about 3 to 4 minutes.

Meanwhile, in a stand mixer fitted with the whisk attachment, beat egg whites, cream of tartar, and salt on medium speed until soft peaks form, about 2 minutes.

Gradually add the remaining 1/4 cup sugar to the egg whites and continue whipping until stiff peaks form.

Remove the syrup from heat and let it cool for 30 seconds, then with the mixer running on medium speed, slowly stream the hot syrup into the egg whites over about 1 minute.

Increase mixer speed to medium-high and whip the meringue until glossy, thick, and cool to the touch, about 3 to 4 minutes.

Add vanilla extract and whip for 30 seconds to incorporate.

Use the Italian meringue immediately for frosting or toppings, or transfer to an airtight container and refrigerate for up to 3 days.