Step 1 Scrape the seeds from the split vanilla bean into milk in a saucepan. Heat the milk just to a simmer.
Step 2 Whisk the sugar into the egg yolks and beat until pale yellow and a thick ribbon consistency. Whisk in the flour.
Step 3Add a little of the hot milk to the egg mixture to temper, then stir in remaining hot milk.
Step 4Strain the hot milk mixture back into a saucepan and return to heat. Heat and stir to boiling. Reduce the heat and simmer about 1 minute or until the pastry cream is shiny and thick.
Step 5Transfer the hot pastry cream to a mixing bowl. Beat on medium speed, adding 1 tablespoon cold butter bit by bit until the custard is cooled. Cover tightly and chill until cold.