Cut pumpkin or squash into 1-inch chunks.

Place pumpkin in a pan, cover with water, and bring to a boil.

Reduce heat and simmer until tender, about 20 minutes.

Transfer to a food processor, reserving cooking liquid.

Puree pumpkin, adding cooking liquid to achieve a smooth consistency.

Pour 2 cups of pumpkin puree into a large bowl and set aside.

Preheat oven to 350 degrees Fahrenheit.

Heat 3/4 cup sugar in a shallow saucepan over medium heat, swirling until melted and amber colored.

Pour the caramel syrup into a 6-cup metal loaf pan, tilting to coat the bottom evenly.

Place the loaf pan on a wire rack to harden the caramel.

Combine remaining sugar, eggs, and egg yolks in a bowl and whisk until blended.

Heat milk until bubbles form around the edges, then whisk into the egg mixture.

Pour the egg mixture through a sieve into the pumpkin puree.

Whisk to combine and pour over the caramel in the prepared loaf pan.

Cover the loaf pan with aluminum foil.

Place the loaf pan in a roasting pan and add hot water to reach halfway up the sides.

Bake until the sides are set and a knife inserted in the center comes out clean, about 40 minutes.

Remove water from the roasting pan and take out the loaf pan.

Cool the flan on a wire rack, then refrigerate for at least 4 hours.

Run a knife around the edge of the flan and invert onto a serving dish to serve.