Place 3 tablespoons sugar and 1 tablespoon water in a small, heavy saucepan over medium heat. Stir only until the sugar dissolves; then stop stirring and swirl the pan occasionally. Cook until the sugar turns a deep amber color (about 6–8 minutes). Watch closely to avoid burning. If needed, gently tilt the pan to even out color.

Once amber, immediately remove from heat and carefully pour the hot caramel into the bottom of the flan mold or divide among ramekins, tilting to coat the bottom evenly. Set aside to harden while you prepare the custard.

Custard

Heat milk and cream

In a medium saucepan, combine 2 cups whole milk and 1 cup heavy cream. Warm over medium heat until just below simmering (small bubbles at the edge), about 5–7 minutes. Do not boil. Remove from heat.

Beat eggs and sugar

In a large bowl, whisk together 4 whole eggs, 2 egg yolks, and 1 cup granulated sugar until smooth and sugar begins to dissolve, about 1–2 minutes. Whisk in a pinch of salt, 1 teaspoon vanilla extract, and optional 1/2 teaspoon citrus zest.

Temper the eggs

Slowly stream about 1/2 cup of the hot milk mixture into the egg mixture while whisking constantly to temper the eggs. Continue adding the hot milk in a slow steady stream, whisking, until fully combined.

Strain custard

Pour the tempered custard through a fine-mesh sieve into a clean pitcher or bowl to remove any cooked egg bits and zest fragments. This yields a silky custard.

Assemble

Pour the strained custard over the set caramel in the flan mold or divide evenly among ramekins (leave a small gap at the top).

Baking and Cooling

Prepare water bath

Place the filled mold or ramekins in a baking pan. Pour hot (not boiling) water into the pan until it reaches about halfway up the sides of the mold/ramekins to create a bain-marie (water bath).

Bake

Preheat oven to 325°F (160°C). Carefully transfer the baking pan with water bath to the oven. Bake until the custard is set but still slightly jiggly in the center, about 40–55 minutes for a 6-cup mold, or 25–30 minutes for individual 6–8 oz ramekins. A knife inserted near the center should come out mostly clean.

Cool

Remove the mold/ramekins from the water bath and cool on a wire rack for 30–60 minutes. Then refrigerate uncovered until thoroughly chilled, at least 4 hours or preferably overnight.

Unmold

To serve, run a thin knife around the edge of the flan to loosen. Place a serving plate over the mold or ramekin, invert quickly and gently. Tap or shake lightly — the caramel will flow over the custard. If caramel sticks, warm the mold base briefly with a blowtorch or dip the bottom of the mold in warm water for a few seconds, then invert again.