Combine egg yolks & sugar in a bowl and whisk them together, adding 50 milliliters of milk to make it easier.
Add the flour and whisk again, forming very uniform, clump free batter. Add another 50 milliliters of milk to make it easier
Heat the remaining milk in a pot until boiling, all the while stirring to keep the bottom from scorching.
Transfer the the egg & flour mixture into a larger pot (about 3 quarts).
Tamper the egg yolk mixture by slowly pouring the hot milk in, all the while mixing.
Pour the mixture back into the pot and bring it to boil over medium heat, continuously (!!) stirring and keeping the bottom of the pot from scorching. Once boiling, cook for 2-3 minutes.
Add vanilla extract & stir. Add butter, allow to melt and stir to for a smooth custard.
If your custard had clumps, run it through a fine sieve or pulse it with an immersion blender (handheld) until smooth.
Allow the custard to cool by pouring it into a jelly roll baking sheet and covering it with plastic wrap (plastic wrap should touch the custard, to keep the skin from forming). Do not refrigerate, just bring to room temperature.