To make the mousseline cream, in a large bowl, combine the egg yolks, sugar, flour, corn starch and salt. Whisk to combine and set aside.
In a sauce pan, warm the milk until it just beings to scald at the edges, making sure not to bring to a boil. Add in the vanilla. Once scalded, slowly drizzle milk into egg yolks, whisking constantly so as not to scramble the eggs (an easy way to do this is to transfer some of the milk to a liquid measuring cup with a spout). Once all of the milk has been transferred to the egg mixture, return everything to the saucepan and heat until it thickens and just begins to boil. Continue to boil for one minute. The mixture should coat the back of a spoon. Remove from heat, pass through a sieve to eliminate any egg pieces or lumps, and pour into a bowl. Immediately cover with plastic wrap, directly on the surface, and set aside to cool to room temperature.
Once the custard has cooled, transfer the room temperature butter to the bowl of a stand mixer and whip on high speed with the whisk attachment for 3-4 minutes or until pale and fluffy. Add the room temperature custard, one tablespoon at a time, whipping for about 20 seconds between each addition to make sure it is incorporated. Once all the custard has been added, whip for an additional one minute. Cover lightly with plastic wrap and keep chilled until ready to use.