Over medium-low heat, combine milk, sugar, cinnamon sticks, and vanilla. Bring to a boil while stirring to dissolve the sugar. Remove it from heat as soon as it begins to boil. Allow to cool off the heat for 30 minutes.
Eggs are whisked in a medium bowl.
A spoonful of the cooled milk mixture should be added to the eggs after the cinnamon sticks have been taken out. To temper the eggs, continue whisking and add a few more teaspoons. After that, add the remaining milk to the eggs and whisk to make a smooth mixture.
Get a large baking dish, fill it halfway with water, then add your empty ramekins. Make sure the water doesn't reach the ramekins' rims. After that, pour the custard mixture into each ramekin.
For the whole milk version, bake for 30 minutes; for the oat milk version, bake for 45 minutes. See if a toothpick can be inserted and removed cleanly. If not, bake longer until the toothpick is clean. Place under the broiler for 4 minutes after completion, or until the tops are browned.
Jericallas should be taken out of the dish and put in the fridge for at least two hours. If desired, garnish with berries and serve in ramekins.