Almost nothing makes me happier than buttercream at the perfect temperature, a small offset spatula, and a beautiful cake waiting to be frosted. There are different types of buttercreams, but I prefer the Italian-meringue version, perhaps because its consistency is very similar to thick oil paint, Thiebaud's medium in Display Cakes. If you're making one of the variations, ensure that whatever you're adding is at room temperature and incorporate it slowly.
Note: This recipe requires an accurate thermometer for taking the temperature of the sugar syrup. Instant-read thermometers are notoriously inaccurate (which is why I love my thermocouple), so if you own an instant-read thermometer, it's good practice to check its calibration before beginning. Simply bring a small pot of water to a boil and verify that the thermometer registers 212°F (at sea level) when inserted into the water. To easily clean a pan or measuring cup that's sticky with the remnants of sugar syrup, fill it with warm water and let it stand until the sugar dissolves, about 30 minutes.
Do Ahead: Stored in an airtight container, buttercream will keep for up to 1 week in the refrigerator or for up to 4 months in the freezer.
Variations: For the Thiebaud Pink Cake substitute Strawberry Concentrate for the vanilla extract, adding it slowly with the mixer running.
Working with Buttercream
In order to make the most beautifully decorated cake, your buttercream must be perfectly smooth and spreadable. Whether you're using a just-made batch or one that has been made ahead and refrigerated, to get it to the optimal consistency for decorating, it's necessary to warm and rewhip it. Here's how.
Place about 2 cups (10.4 oz / 292 g) of buttercream in a microwavable container. Heat it for no more than 5 seconds at a time at full power until it begins to look slightly glossy but not at all melted; it should still be solid, with a sheen from a softened exterior—not unlike ice cream on a cone as it begins melting and dripping onto your hand. (It's very easy to overheat and melt buttercream, so it's better to warm only a couple cups at a time and to use short bursts in the microwave.) Beat the warmed buttercream in the bowl of a stand mixer fitted with the paddle attachment until it resembles mayonnaise, about 30 seconds. Repeat as needed.
