Preheat oven to 425 degrees. Combine the water and butter in a large pot and bring the mixture to a boil. Add in the flour and salt, stirring until the mix takes on the shape of a ball. Move the dough ball into a bowl, and rapidly beat in the eggs, taking care to mix quickly as to not cook the eggs. Use two spoon to roughly mold and transfer the dough to an ungreased baking sheet. Be sure not to use an ice cream scooper, because it will cause your pastries to be too dense and take on a doughnut-hole texture. Bake the pastries for 22-25 minutes until puffy and golden. While still in the oven, use a toothpick or chopstick to poke a whole in each of the pastries. This will allow the hot air to escape, and keep the cream puff from collapsing. Allow to cool completely.
To make the egg yolk custard, combine the flour and cornstarch in a small bowl and set aside. On medium heat, combine the milk, vanilla, and butter, allowing it to simmer, but stirring constantly to keep from burning the milk and creating lumps. Beat the eggs and sugar together until frothy. Stir in the mixture, whisking constantly. When combined, add in the flour and cornstarch. Stir constantly until thickened. Add more sugar and vanilla to taste, you want the custard to be sweet.
Allow the custard to cool completely. When cool, cut the pastry almost in half. Use a piping bag to pipe the desired amount of custard into the pastry. Add in cut strawberry halves, then pipe just a little more custard to the top to hold the pastry together. Garnish by drizzling melted bakers chocolate over the top with a fork and a dusting of powdered sugar.
Best served slightly chilled!