Preheat the oven to 350º F.
Grease and flour a 10" flan pan.
Sift the flour, cornstarch, and baking powder into a large mixing. In a separate bowl beat 4 eggs and the melted butter.
Add the wet ingredients to the dry ingredients.
Mix with a wooden spoon until blended.
Pour into the prepared pan and bake for 30 minutes.
Note: The original recipe calls for 1/4 cup of all-purpose flour, 1/2 cup cornstarch, 1/2 cup sugar, 2 teaspoons baking powder, 4 1/2 tablespoons melted butter,and only 2 eggs. Given that I like to use a large tart tin and prefer a thicker cake base (about 2 inches high), the recipe is doubled.
In a saucepan, bring to boil the sugar and water.
When the sugar is dissolved, remove from the heat.
Pour the milk and vanilla into a saucepan and heat to just below the boiling point.
Remove from the heat and let stand for 15 minutes to allow the vanilla flavour to infuse.
Strain the milk and reheat.
In a mixing bowl, whisk the egg yolks and sugar until pale and thick.
Whisk in the flour until well incorporated into the wet ingredients.
Gradually whisk in the hot milk until blended.
Transfer to a heavy-based saucepan and whisk continuously, over medium heat until the custard thickens and begins to boil.
Simmer and continue whisking for an additional two minutes to allow the flour to cook.
Remove from the heat and blend in the butter. Whisk in the Galliano liqueur or liqueur of choice if desired.
Transfer to a bowl and place a piece of plastic wrap directly on the surface of the custard to prevent a film from forming.
Note: Normally, the knob of butter is rubbed on the surface of the cooked custard to prevent a skin from forming. I prefer using the plastic wrap for that purpose and integrate the butter into the custard for extra flavour. I feel it also gives a much smoother finish to the custard.
The recipe makes more than enough to cover the cake surface. I ended having about half a cup of the crème pâtissière leftover.