In a saucepan, mix sugar, cornstarch and salt. Gradually stir in milk. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute.
Gradually stir at least half of the hot mixture into egg yolks, then stir back into hot mixture in saucepan. Boil and stir 1 minute; remove from heat. Stir in butter and vanilla.
Store covered in refrigerator.
While waiting for pudding to cool, sugar berries. Rinse fruit and place in bowl. Pour a corn syrup over fruit and gently stir until coated. Pour sanding sugar and sanding sprinkles over fruit until coated in sugar.
Using a cookie scoop, place pudding in cooled cookie tarts. Use a spatula to smooth out the top of the tarts.
Finish tarts by adding fruit and flower decoration. Place larger berries on the tarts first, staying on just one side of the tart. Fill in with smaller berries. Use a mix of sugared and non-sugared berries for a more textured look. Add in tiny flowers to the berries and tuck in mint leaves or sprigs of greenery.
Store in refrigerator until ready to serve.