Pastry chefs generally disdain this frosting, sometimes calling it "faux buttercream" because it is not cooked like European buttercreams. BUT for all of their disdain, the pull of nostalgia can be strong indeed, since many Americans seem to prefer this to the others. Look at famous bakeries like Magnolia and Sprinkles, that have made a name for themselves using variations of this simple retro butter frosting. They have lines around the block for their cupcakes!
My own grandmother made the frosting recipe on the back of the Domino Sugar box, which called for 1 pound of confectioner's sugar to 1 stick of butter, but the modern consensus seems to be that increasing the butter to sugar ratio makes it tastier. Another change I made for my own recipe is that I like to beat mine in my mixer for several minutes (up to ten minutes sometimes), which makes it very fluffy. This frosting is also infinitely changeable - you can add chocolate, instant coffee powder, or various extracts to get different flavored frosting, so feel free to experiment!
4 Cups Confectioner's sugar (1 lb.)
1 Cup good quality Butter (I use salted butter for this since I like a little bit of salt)
2 tsp Vanilla extract
1 Tablespoon milk or heavy cream (use slightly more for a softer consistency)
Place the butter and sugar in a mixer and beat on low speed until combined. (If using a KitchenAid mixer, use the paddle attachment, not the whisk). Add vanilla and milk, then beat on low speed until it looks very fluffy. Be patient - I let this go in the mixer for 6-8 minutes. Mixing it for several minutes helps to make it fluffy and minimze the "grittiness" of the powdered sugar. If needed, add additional milk one teaspoon at a time until you get the consistency you want - thicker is better for piping swirls onto cupcakes; softer is better for frosting cakes.