Step 1

Preheat oven to 325°. Stir 1/4 cup sugar and 2 Tbsp. water in a medium heavy saucepan over medium-low heat until sugar dissolves. Increase heat and boil, occasionally swirling pan and brushing down sides with a wet pastry brush, until caramel is medium-dark amber, 5–6 minutes. Working quickly, divide caramel among ramekins, coating bottoms evenly. Let cool until hard.

Step 2

Gently whisk remaining 1/3 cup sugar and eggs in a medium bowl. Add cream, half-and-half, and vanilla; gently whisk to combine (do not aerate). Let sit until sugar dissolves, about 5 minutes. Strain through a fine-mesh sieve set over a medium pitcher to eliminate air pockets.

Step 3

Place ramekins in a roasting pan lined with a kitchen towel; place pan on a large rimmed baking sheet. Divide custard among ramekins. Pour hot water into roasting pan to reach a little more than halfway up sides of ramekins. Cover pan with foil, crimping to seal. Carefully transfer to oven; cook for 25 minutes. Rotate pan. Cook until custard is set but center still jiggles when dish is nudged, 20–25 minutes longer.

Step 4

Uncover roasting pan. Let flan cool in pan for 30 minutes. Remove ramekins and let cool completely. Cover with plastic; chill overnight.

Step 5

Let stand at room temperature for 10 minutes before serving. Unmold flans onto plates.