Beat the syrup into the yolks in a steady stream. Don’t allow the syrup to fall on the beaters or they will spin it onto the sides of the bowl. Continue beating on high speed for 5 minutes. Allow it to cool completely. To speed cooling, place the buttercream in an ice water bath or the refrigerator, stirring occasionally. When the outside of the bowl feels cool, on high speed, beat in the butter by the tablespoon. The buttercream will not thicken until almost all of the butter has been added. On low speed, add the lemon zest, lemon juice, and vanilla. Raise the speed to high and beat until smooth and creamy. It may separate at first but come together on beating.