Prepare an ice bath by adding ice and water to a large bowl. Have a medium bowl with a fine-mesh sieve ready nearby.
In a small 1.5-quart saucepan, add half and half, 1/3 of sugar and vanilla bean or paste. Heat on medium low until mixture is hot and one or two small wisps of steam just begin to rise, about 5-6 minutes. Do not bring to a boil.
Meanwhile, place a medium bowl on top of a wet paper towel (this will give you stability when whisking) and in the bowl, whisk together egg yolks and 1/3 sugar until thick. In a small bowl, sift together flour and remaining 1/3 of sugar. Then pour into the yolk mixture and whisk until combined. Mixture will be very thick.
When half and half is hot, slowly stream about ½ cup (just eyeball it) into the egg yolks mixture while whisking quickly. This will gently raise the temperature of the eggs, so they don’t scramble.
Pour the mixture back into the saucepan and continue cooking for about 7-8 minutes on medium low, while whisking constantly, paying special attention to the corners of the pan. Custard will begin to thicken and have a sheen to it. You can tell when the custard is properly thickened when the whisk begins to leave a trail, and the mixture holds its shape when dropped from the whisk.
Immediately remove from the heat and strain through the prepared fine-mesh sieve into the medium bowl. Press on the custard with a spoon or spatula until pressed through. Stir in butter while warm, then place in an ice bath, stirring occasionally until smooth. Once cooled, place plastic wrap directly on the surface of the pastry cream and chill in the refrigerator until ready to assemble.