In a medium saucepan (off heat), whisk together yolks, sugar, cornstarch, lemon zest, salt and ½ cup of milk until smooth. Whisk in remaining 2 cups milk.
On medium heat, bring mixture to a gentle simmer, whisking constantly. Once it reaches a simmer, the mixture will thicken. Pull off heat when it is thick enough to coat the back of a wooden spoon.
Pour mixture into a mixing bowl and place a layer of plastic wrap directly on top. Refrigerate until pudding cools completely, 2-3 hours, before serving.
Makes about 4 ½ cups of pudding.
To serve, spoon about 1 tablespoon of chilled lemon pudding into each RomaCrunch cup. Top each with a blackberry and sprinkling of roasted pistachios and lemon zest. Serve cold.
Note: Recipe makes approximately 20 pieces