Vegan custard tart

Name: Vegan custard tart
Prep time: 30 min
Cook time: 35 min
Total time: 1 h 5 min
Cuisine: French
Ingredients: 210 g / 1¾ cups all purpose flour or GF plain flour mix; 25 g / 2 tbsp caster (superfine) sugar; ½ tsp fine salt; 1½ tsp xanthan gum (only if using GF flour); 100 g / 3.5 oz vegan butter* (about ½ cup), cubed; approx. 3 tbsp-4 tbsp ice cold water; 55 g / scant ½ cup cornflour / cornstarch; 750 ml / 3 cups + 2 tbsp creamy plant milk*; 125 g / ½ cup + 2 tbsp sugar; 15 ml / 1 tbsp vanilla extract; 1/3 tsp ground cardamom*, optional; 2 pinches of kala namak (eggy salt) or regular salt; 70 g / 2.5 oz (about 1/3 cup) vegan butter; a sprinkle of nutmeg or cinnamon, optional; 400 g pink rhubarb; 35 g / 3 tbsp caster (superfine) sugar