1) For the syrup: Combine the sugar and water in a small, nonstick (preferable, if you have one) saucepan. Bring to a boil over medium heat, stirring occasionally until the sugar dissolves. Cook, without stirring, until the syrup reaches at least 240°F. It can go as high as 248 to 250°F. Just be sure it's within those temperatures before you take it off the stove.
2) For the meringue: While the syrup is cooking, combine the meringue powder, water, and salt in the bowl of your mixer. Beat at high speed with the whisk attachment until the mixture first looks foamy, then turns white, and you begin to see tracks in the bowl. At this point, slowly sprinkle in the sugar with the mixer running. Increase the speed to high and beat until the mixture is stiff.
3) When the syrups gets above 240°F (110°C) and before it gets above 250°F (121°C), remove it from the heat and with the mixer running at low speed, pour it down the side of the mixing bowl(not on the whisk or beaters if you can avoid it; that will send the syrup flying and start spinning sugar threads instead of incorporating it). Once the syrup is all in, leave the mixer running until the mixture cools to 80°F. You can help this process along by wrapping an ice pack around the mixing bowl.
4) Once the meringue is cool, add the butter, a few pieces at a time, with the mixer running at medium to medium-high speed. The meringue will deflate a bit, and may begin to look curdled. Don't lose heart! This ugly "adolescent" stage is normal. Just keep the mixer running and adding the butter.
5) Soon the frosting will begin to bring itself together around the whisk, then in the rest of the bowl. Once most of the butter is in, add vanilla or your choice of flavorings. If you're using the frosting for decorations, add the vegetable shortening in chunks at this point.
6) Use the buttercream within 4 hours, or refrigerate until needed.