Combine all ingredients except for butter and vanilla in a saucepan. Blend with an immersion blender until completely smooth.
Cook over low to medim heat stirring occassionally, the mixture will start to thicken. The idea is to heat the mix up slowly and evenly.
Once the mixture becomes thick, you will need to stir more frequently, I like to mix with a whisk, keeping a spatula handy the get into the corners once it's thickened up.
Bring to a boil, stirring frequently, let cook for 2 to 3 minutes, taste to ensure that the cornstarch has cooked out completely.
Pass through a sieve into a bowl (if you stirred well you shouldn't have any lumps but this is to catch any bits of the ackee that might have missed the blade of the immersion blender)
Whisk in butter piece by piece until completely melted, then add vanilla.
Pour into prepared tart shell and press a piece of plastic directly on top the pastry cream. Chill till set (about 4 hours). Once the cream is set you can finish your tart as desired.
