Unsalted butter, softened, for ramekins or bowls
¾ cup whole milk
⅓ cup heavy cream
2 large eggs plus 1 large yolk
Kosher salt
6 tablespoons corn kernels (from 1 cob)
Preheat oven to 300°F. Butter six ramekins; place in a baking dish large enough to hold all of them. In a small saucepan, bring milk and cream to a simmer. In a heatproof bowl, whisk together eggs and yolk; season with 1/4 teaspoon salt. Gradually whisk hot liquid into egg mixture. Divide mixture evenly among prepared ramekins. Add 1 tablespoon corn kernels to each ramekin.
Place baking dish on oven rack and add boiling water until it comes halfway up sides of ramekins. Bake until custards are just set along edges but still wobbly in centers, about 30 minutes. Remove ramekins to a wire rack; let cool 15 minutes, then refrigerate, covered, at least 6 hours and up to 2 days.
When ready to serve, run a paring knife around edges of each custard, then dip bottoms into a bowl of hot water to loosen, about 30 seconds; invert into serving bowls.