Vanilla: Combine yols, sugar, flour and cornstarch in a medium saucepot. Add milk, heat to medium high, stirring constantly. When the mix boils, turn it to low, and stir for 2 minutes or so until it thickens. Pour into prepared pastry shell or layer with other trifle ingredients. Lemon: Add ingredients to a saucepan, heat on medium high, stirring constantly, until it boils. Cut heat to low and stir until the mixture thickens - about 2 minutes. Ganache: Bring cream to a boil, remove from heat and stir in chocolate. Mix until blended.