1. In a small bowl, add sugar, egg yolk and mix well. Sift the flour in the bowl to combine them. Mix well.
2. Pour the milk gradually onto the egg mixture. Mix well until combined evenly.
3. Strain the mixture to saucepan using a strainer and cook over moderate heat, stirring constantly, until sauce thickens, using a wooden spoon or a spatula. When the sauce thickens enough, add maple syrup and vanilla paste. Blend well.
4. Let cool at room temperature. It will become firmer when cold.