Preheat oven to 180°C (thermostat 6). Butter a 22 cm diameter tart pan. Roll out the pastry and line the pan, ensuring the base and edges are well covered. Refrigerate the pan while preparing the custard.

In a medium saucepan, bring the milk, half the granulated sugar (100g) and the vanilla sugar (or vanilla bean seeds) to a temperature close to boiling over medium heat, without letting it boil. Remove from heat.

In a large bowl, whisk the whole eggs vigorously with the remaining granulated sugar (100g) until the mixture whitens and becomes foamy. Add the cornstarch and heavy cream. Mix vigorously until a smooth mixture is obtained.

Gradually pour the hot milk into the egg-sugar-cornstarch mixture, whisking constantly and without interruption to prevent the eggs from coagulating. Mix well until perfectly smooth.

Gently incorporate the pistachio paste into the mixture. Mix well until a uniform and smooth consistency is obtained.

Return the preparation to the saucepan. Cook over low heat, stirring constantly for about 5 minutes, until the custard thickens and coats the spoon. Ensure that it does not stick to the bottom of the pan.

Pour the pistachio custard over the pastry in the tart pan.

Bake for approximately 50 minutes. The flan is cooked when the top is golden brown and the center is still slightly wobbly.

Let the flan cool at room temperature for 30 minutes. Then, refrigerate for at least 4 hours, or overnight, to allow the flan to set completely and the flavors to develop. Serve chilled.