Wipe down inside of mixing bowl and whisk attachment with generous amount of white vinegar. Wipe out any residule vinegar. 

Separate egg whites. Add dash of cream of tartar And salt. Start mixing on high speed.

Add 1 3/4 cup sugar to small pot along with 1/2 cup water. Put on med-high heat. Attach thermometer and boil until 240-250F. Anywhere in between this temperature is fine. I usually bring to a boil, and keep on medium-low heat just below 240F to give meringue enough time to get stiff. Stir sugar syrup in the begging to keep the sugar from burning.

While the eggs whites are whipping and a little frothy, slowly add in about 1/4 cup sugar and whip to STIFF peaks.

When meringue is ready, bring sugar syrup up to 240-250F by turning up the heat. Once temperature is reached, slowly add the syrup to egg white. Carefully pour the hot syrup into the egg white directly while whisking on HIGH speed. DO NOT POUR SYRUP DOWN THE SIDE OF THR METAL BOWL- this will cool down the syrup. Pour syrup in between whisk and side of the bowl so that it directly touches the egg whites.

Whisk meringue for 5-10 mins until stiff peaks form. The bowl will be hot. Bring meringue to room temp by placing either a bag of ice, frozen peas, or cold towel around the metal. Once completely cooled, you are ready to add the butter to the meringue.

Add the room temperature butter in 1-2 tablespoon increments at a time. If meringue buttercream turns soupy, put the entire mixer bowl and whisk in fridge for 15 mins and rewhip for about 10 minutes.