You can whip egg whites to stiff peaks, but you cannot whip whole eggs or egg yolks to true stiff peaks in the same way. Stiff peaks are mainly an egg white thing.

Egg whites are mostly water and protein, with almost no fat. When you whip them, air gets trapped inside the liquid. The proteins unfold and link together around those air bubbles, creating a stable foam. As you keep whipping, the foam becomes thicker and stronger. Eventually, it reaches soft peaks, firm peaks, and then stiff peaks.

Stiff peaks mean the whipped egg whites stand straight up when you lift the whisk. They hold their shape without drooping much. This is what you want for meringues, soufflés, angel food cake, macarons, and other recipes that rely on a strong, airy structure.

Egg yolks are different. Yolks contain fat. Fat interferes with the protein structure needed to make a stable foam. Even a small amount of yolk in egg whites can stop the whites from whipping properly. That is why recipes often tell you to separate eggs carefully and make sure no yolk gets into the whites.

This does not mean egg yolks cannot be whipped at all. Egg yolks can be beaten until they become thick, pale, and creamy. When whipped with sugar, yolks can reach what is called the ribbon stage. This means the mixture falls from the whisk in a thick ribbon that briefly sits on the surface before sinking back in. This is common in custards, sponge cakes, sabayon, and some dessert bases.

Whole eggs can also be whipped, especially with sugar. They can become light, foamy, and increased in volume. This is often done for sponge cakes and genoise. However, whole eggs do not usually form stiff peaks because the yolk fat weakens the foam. They can become airy and thick, but not as dry, firm, and structured as whipped egg whites.

The simple rule is this: egg whites whip to peaks, yolks whip to ribbons, and whole eggs whip to a foam.

For best stiff peaks, use clean egg whites with no yolk mixed in. Make sure the bowl and whisk are completely clean and free of grease. A metal or glass bowl is better than plastic because plastic can hold onto oils. Room-temperature egg whites usually whip with better volume, though cold eggs are easier to separate. A small amount of acid, such as cream of tartar or lemon juice, can also help stabilize the foam.

Do not overwhip egg whites. Once they pass stiff peaks, they can become dry, grainy, and separated. Properly whipped egg whites should look glossy and smooth, not clumpy or broken.

So, can you whip egg white and yolk together until stiff peaks form? Not really. Whole eggs can get foamy and thick, but true stiff peaks come from egg whites alone. If a recipe calls for stiff peaks, it means separated egg whites, not whole eggs and not yolks.