Place the sugar and egg whites in the bowl of an electric mixer. Whisk to combine, and set aside.

Prepare a double boiler: Set a medium-sized saucepan filled with a cup or two of water over high heat. When the water comes to a strong simmer, reduce the heat to medium-low. The water should stay at a steamy simmer, not a boil.

Place the mixing bowl with the sugar-egg white mixture over the pot and, whisking frequently as to not cook the egg in spots, heat until very warm. The mixture should be hot to the touch but not burning, and you should no longer feel any grains of sugar.

Turn the heat off and transfer the mixing bowl to the mixer fitted with the whip attachment. Whip the meringue on medium-high until they form medium-stiff peaks and cool to about room temperature, approximately 10 to 15 minutes.

Switch the mixer to a paddle attachment. With the mixer on medium-low, add the softened butter in large pieces (I usually do about ⅓ or ½ stick at a time). When all the butter has been added, raise the mixer to medium speed and beat until fluffy and smooth, about two minutes.PRO TIP: If the buttercream is too runny, that means it is too warm. Hold a couple of bags of frozen vegetables to the side of the bowl to cool it, while continuing to beat the buttercream. You can also put the whole bowl in the fridge for several minutes, then whip.PRO TIP #2: If the buttercream is clumpy, it means that the butter was too cold when added. Briefly place the entire bowl over the double boiler to raise the temperature and melt the butter a little bit. Try to rewhip. Alternatively, take out a scoop of buttercream, microwave it until just melted. Then add that into the buttercream, and whip. The hot buttercream should raise the overall temperature.

With the mixer on low, add the vanilla and salt. Combine well. The buttercream is ready to use.

To make this raspberry buttercream: Add ⅓ cup seedless raspberry jam and a squirt of lemon juice after the vanilla and salt. A drop or two of red food coloring (preferably natural) may also be added to give it a brighter pink hue.

To make this chocolate buttercream: Add ⅓ cup melted and cooled (not hot) pure, semisweet chocolate (between 58% to 72% cacao), after the vanilla and salt. PRO TIP: Do not use chocolate chips because they contain too many emulsifiers and other additives.

To make this lemon buttercream: Add the zest of two lemons and the juice of ½ a lemon after the vanilla and salt.