This is a staple of traditional pastry shops. A delicious base that you can flavour with whatever strikes your whimsy. This is from Julia Child's bible, 'The French Chef Cookbook'.
French Pastry Cream
6 egg yolks
A heavy bottomed 21/2 quart stainless-steel or enameled saucepan
1/2 cup granulated sugar
1/2 cup all-purpose flour
2 cups hot milk
2 TBS butter
1 TB vanilla extract or 1/2 TB vanilla and 1 or 2 TB run or Kirsch or....
Place the egg yolks in the saucepan, and with a wire whip gradually beat in the sugar. Continue beating for a minute or two until the mixture is thick, pale yellow, and forms ribbons.
Beat in the flour, then beat in the hot milk in a thin stream. Set over moderately high heat and stir slowly and continuously with a wire whip, reaching all over bottom and sides of pan, until mixture thickens. As it turns lumpy, beat vigorously to smooth it out.
Lower the temperature and continue stirring for several minutes to cook the flour and thicken the cream. Be very careful about scorching cream in bottom of pan; be sure your pan is heavy, be sure to keep stirring, and do not use high heat, particularly after cream has started to thicken.
Remove from heat; beat in butter and the flavouring. Clean off sides of the pan with a spatula. Film top of the pastry cream with 1/2 TBS milk, rum or kirsch to prevent crusting. Chill. (Pastry cream will keep 3 or 4 days under refigeration or may be frozen.)