In a 3 quart saucepot combine the milk and vanilla bean seeds. Combine the yolks and sugar in the bowl of a standing mixer fitted with the whisk attachment. Sift the cornstarch onto a piece of wax or parchment paper.
Scald the milk. Meanwhile, beat the yolks and sugar until they lighten in color. Remove the bowl from the mixer and stir in the cornstarch. Pour about 1/3 of the hot milk into the egg mixture, whisking continuously to incorporate. Whisk the tempered egg mixture back into the remaining hot milk, and place the pot back on the heat. Cook on medium high heat, stirring constantly but gently with the whisk, until the mixture comes to the boil. Reduce heat and let it boil for about 30 seconds, stirring constantly. Remove from heat and gently whisk in the butter. Strain the pastry cream into a bowl, place plastic wrap right down on the surface, and chill completely. This can be made up to 3 days ahead.