Brown Sugar Buttercream - cooking recipe

Ingredients

3 large egg whites, at room temperature
3/4 teaspoon salt
1 cup dark brown sugar, packed
1/2 cup water
1/2 teaspoon lemon juice, fresh is best
1 1/2 cups butter, cut into pieces and softened
2 teaspoons vanilla

Preparation

Combine egg whites and salt in a large bowl, and just let it sit for now.
Stir together brown sugar and water in a small heavy saucepan and bring to a boil over moderately high heat, washing down the sides of the pan with a pastry brush dipped in water. This brings the bubbles down and stops them from boiling over the pan.
When sugar syrup reaches a boil, start beating egg whites with an electric mixer at med-high speed until frothy, then add the lemon juice. Beat at med. speed until whites just hold soft peaks. DO NOT BEAT AGAIN UNTIL SUGAR SYRUP IS READY.
Meanwhile, put candy thermometer into sugar syrup and continue boiling until syrup reaches 238-242\u00b0. Immediately remove from heat and pour into a heatproof 1-cup glass measure.
Beating the egg whites at high speed, slowly pour the hot syrup in a thin stream down the side of the bowl into the egg whites.
Beat meringue, scraping down bowl, until meringue is cool to the touch,about 6-7 minute IT IS IMPORTANT THE MERINGUE IS PROPERLY COOLED BEFORE PROCEEDING.
With mixer at medium speed, gradually add butter 1 piece at a time, beating well after each addition until incorporated. (If meringue is too warm and buttercream looks soupy after some of the butter is added, briefly chill bottom of bowl in a large bowl filled with ice water for a few seconds before continuing to beat in remaining butter).
Continue beating until buttercream is smooth. (Mixture may look curdled before all of the butter is added, but it will come back together before beating is finished).
Add vanilla and beat 1 more minute.