Vinegar can replace the lemon juice if desired.
If you don't have a candy thermometer, simmer the egg whites and white sugar until the sugar has completely dissolved and the egg whites are hot. You can feel a drop between your fingers to ensure no granules are left.
You can use a bowl and whisk of you don't have a stand mixer. Wipe both with lemon juice.
If buttercream is too runny, the butter was possibly too soft. Place into the refrigerator for about 15 minutes, then beat again. If still too runny, add a few more cubes of butter and keep beating until it reaches desired consistency.
For chocolate buttercream, melt 1 1/2 cups bittersweet chocolate. Add to every 5 cups buttercream and beat until incorporated.
For strawberry buttercream, add about 1/2 cup strawberry puree to 5 cups buttercream and beat until incorporated.
For vanilla bean buttercream, add 1 tablespoon vanilla bean paste or 1 vanilla bean for every 5 cups buttercream and beat until incorporated.