For the Pastry:
Preheat oven to 400 degrees F. Grease 2 large baking sheets.
In a medium saucepan, combine water, butter and salt. Bring to a boil then remove from heat. Add flour and, using a wooden spoon, stir vigorously until combined and smooth. Return the mixture to low heat, and stir continually until the mixture pulls away from the side of the pan and forms a ball, about 1 to 2 minutes.
Remove the mixture from heat. Beat in eggs, one at a time, stirring vigorously until smooth after each addition. Drop batter by scant 1/4 cupfuls about 3 inches apart onto prepared cookie sheet.
Bake 35 to 40 minutes until puffed and golden Puffs should sound hollow when lightly tapped.
Let cool completely on wire rack.
To assemble, with a sharp knife, cut off tops crosswise. Remove any soft dough from inside the pastry. Fill with choice of filling. Replace top. Glaze with melted chocolate or dust with confectioner's sugar.
For Pastry Cream Filling:
In a large, heat safe, mixing bowl, combine cornstarch and 1/2 cup milk and stir to dissolve. Beat whole egg and yolks into mixture. Set aside.
Combine remaining milk with sugar and salt in a saucepan and bring to a boil over medium heat. Remove sauce pan from heat. Set aside and cool milk mixture for 5 minutes. Slowly add egg mixture, in a thin stream, into hot milk.
Return mixture to stovetop and stir vigorously until mixture comes to a boil. Continue to stir until pastry cream becomes thick, about 5-7 minutes. Once pastry cream reaches this stage, remove sauce pan from heat and stir in butter and vanilla extract. To prevent over cooking from residual heat, transfer the saucepan to the ice bath. Stir occasionally until pastry cream is cool about 30 minutes.
Chocolate Cream Filling:
Stir together sugar, cocoa and cornstarch in medium saucepan; gradually add milk and instant coffee. Cook, stirring constantly, until mixture boils; boil and stir 1 minute. Remove from heat.
Gradually stir small amount of hot mixture into egg yolk, blending well; return mixture to pan. Heat just to boiling, stirring constantly.
Remove from heat; stir in butter and vanilla. Pour into bowl; press plastic wrap onto surface. Refrigerate until cold.