Preheat oven to 350°F. Butter six 4-ounce ramekins or custard cups.

Melt 2 teaspoons of butter in a skillet over medium heat. When frothy, add corn and cook, stirring occasionally, for 2 minutes. Season with salt and pepper and transfer to a bowl. Set aside.

Melt the remaining butter in the same skillet over medium heat. When frothy, add the mushrooms and cook, stirring occasionally, for 2 minutes. Season with salt and pepper and transfer to the bowl of corn. Stir together and set aside.

In a 2-cup measuring cup with a pour spout (preferably) or bowl, whisk together eggs and egg yolk, add cream, and whisk until uniform. Add salt, pepper, and garlic; stir to combine.

Fill each ramekin halfway with egg mixture, then divide the corn and mushroom mixture among the ramekins, and fill with remaining egg mixture.

Place ramekins in a bain-marie, add hot water until it's halfway up the sides of the ramekins.

Bake for 20 to 30 minutes, or until the custard is just firm in the center.

Remove from oven and let stand in the water bath for 20 minutes. To unmold, run a knife around the edge of the custard and invert.