In a heatproof bowl, mix sugar and egg whites. Make sure the bowl is large enough to fit over a saucepan for the next step.

Place the bowl over a saucepan of simmering water, ensuring the bottom of the bowl doesn’t touch the water.

Gently stir the mixture with a spatula or whisk (without whipping) until the sugar dissolves completely.

To check, rub a bit of the mixture between your fingers, it should feel smooth with no sugar grains.

Remove and Thicken: Once the mixture is warm and viscous, remove the bowl from the bain-marie.

Using a stand mixer fitted with a whisk attachment, beat the mixture on medium-high speed until stiff, glossy peaks form. The bowl should feel cool to the touch at this stage.

Replace the whisk attachment with a paddle (scraper) attachment.

Begin beating the meringue on low speed, adding room-temperature butter a few tablespoons at a time.

If the mixture looks curdled or sticks to the paddle, don’t panic, this is normal! Continue beating, and it will come together into a smooth buttercream.

Once the butter is fully incorporated, add vanilla extract and any desired food coloring. Beat just until evenly combined.

The result is a silky, stable buttercream that’s perfect for frosting cakes, piping onto cupcakes, or filling macarons.