If you’re using a whole vanilla bean cut the vanilla bean in half, and scape the seeds out.
Place the milk, butter, and whole vanilla bean and seeds in a medium sized saucepan over medium heat.
Cook for about 3-5 minutes, stirring consistency and bring to boil. Watch carefully as the milk can overflow if cooked too high/fast.
Once it reaches a boil (about 180-185℉), immediately remove from heat and set aside to cool about 20 minutes.
While the milk is cooling, sift together flour, powdered sugar, salt.
Gently mix the eggs and egg yolks together. You’re trying keep too much air from getting into the eggs so don’t beat too intensely. Then, strain or pour into the flour mixture (no need to mix together at this point).
Once the milk has cooled, (to be exact you want it less than 120℉ so it’s cool enough that it doesn’t cook the eggs), remove the vanilla bean and place in a large bowl. Do not discard the vanilla bean. We’ll be using the vanilla bean and bowl later.
Pour milk mixture onto the flour/egg mix and gently stir together. It will have visible lumps at this point.
Pour the batter over a fine mesh strainer into the large bowl you’ve put the vanilla bean into. Press any lumps out of the batter through the stainer so you get a nice, lump free batter.
Stir in the rum and vanilla extract or vanilla bean paste if using.
Cover the bowl with plastic wrap and place in the fridge for 2 days (a full 48 hours), stirring occasionally to break up the batter. During this time it might get a small film on the top. This is fine and normal. Just stir to redistribute the batter.