Dress up sugar snap peas with a buttery sauce that’s smooth and velvety thanks to the power of emulsions.
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In large saucepan, bring water to boil over high heat. Place snap peas in steamer basket. Use oven mitts to carefully lower basket into saucepan (ask an adult for help).
Use oven mitts to remove lid and lift steamer basket out of saucepan (ask an adult for help). Place steamer basket on large plate.
Slide saucepan to cool burner and let cool slightly, about 3 minutes. Hold saucepan handle with oven mitts and carefully pour water from saucepan into liquid measuring cup (ask an adult for help).
Add shallot, vinegar, salt, and sugar to saucepan. Whisk until sugar and salt are dissolved. Cook over medium heat until mixture is bubbling, 1 to 2 minutes.
Reduce heat to low. Add 1 piece butter to saucepan. Whisk constantly until butter is melted and incorporated, about 45 seconds. Repeat with second piece butter, whisking until sauce is thick and smooth, about 1 minute.
The key to this velvety sauce is not just butter—it’s cold butter added slowly to the pan. What?! No surprise, but this sauce is an emulsion (see the Shake Things Up experiment to learn more about emulsions). It’s made from water and butter, which normally don’t like to mix—that’s why we whisk the sauce constantly as we make it. But butter itself is a mixture of fat and water, too. If all the butter were added to the pan at once, allowing it to melt unchecked, it would separate into greasy fat and water. But slowly adding the cold butter into the hot water—with constant whisking—breaks up the tiny fat droplets as they melt, turning the sauce into a creamy and thick emulsion.