Sift 3 tablespoons of chickpea flour into a small container and then add 2 tablespoons of water and stir well until all of the lumps are out.
Add the remaining 1 tablespoon of water to the chickpea flour mixture and stir again until it's very smooth. Allow it to sit for at least 10 minutes.
Make the custard base
Sift 3 tablespoons of corn starch into a heavy-bottomed saucepan. (Sifting will help avoid lumps.)
Add 3/4 cup of sugar, 1/8 teaspoon of salt, and slowly pour in 1/2 cup of cold oat milk and stir well with a whisk until all of the lumps are gone and the cornstarch is fully dissolved. Then add the remaining 1 1/2 cups of cold oat milk and stir again.
Turn on the heat to medium and stir frequently with a whisk. As it starts to boil slowly, stir constantly.
Once the mixture has boiled slowly for about 30 seconds, slowly add the chickpea flour egg mixture while continuing to stir with the whisk.
Once all of the chickpea "egg" has been added simmer at a low boil for 3 minutes stirring constantly.
After cooking
Remove the custard from the heat and add 4 tablespoons of vegan butter and 2 teaspoons of vanilla extract and stir well until it's incorporated.
Pour the custard into a bowl and lay a sheet of plastic wrap directly over it so that no air is touching it. (This will prevent it from getting a skin on top.)
Eat the warm custard or refrigerate it for at least 4 hours before using it as pastry cream.
Notes
Be sure to add a small amount of liquid to both the chickpea egg and the cornstarch mixture and stir well before adding the remaining liquid. This will help you avoid clumps.
Add the chickpea flour egg to the custard slowly while stirring with a whisk. If the mixture is allowed to settle to the bottom, it will harden before it gets incorporated.