To make the eclairs: Preheat the oven to 425 degrees F. In a large saucepan, bring the water, butter, salt, and granulated sugar to a rolling boil over medium-high heat. When it boils, immediately take the pan off the heat. Stirring with a wooden spoon, add all the flour at once, and stir hard until all the flour is incorporated, 30 to 60 seconds. Return the pan to the heat and cook, stirring, 30 seconds to evaporate some of the moisture.
Scrape the mixture into a mixer fitted with a paddle attachment. Mix at medium speed. With the mixer running, and working 1 egg at a time, add 3 of the eggs, stopping after each addition to scrape down the sides of the bowl. Mix until the dough is smooth and glossy and the eggs are completely incorporated. The dough should be thick, but should fall slowly and steadily from the beaters when you lift them out of the bowl. If the dough is still clinging to the beaters, add the remaining egg and mix until incorporated.
Using a pastry bag fitted with a large plain tip, pipe the dough onto the baking sheet, in 3-inch long segments. Whisk the remaining egg with 1 1/2 teaspoons water. Brush the surface of the dough with the egg wash (you may not use all the egg wash). Bake 15 minutes, then reduce the heat to 375 degrees F, and bake until puffed up and light golden brown, about 25 minutes more. Try not to open the oven door too often during the baking. Let cool on the baking sheet.
Meanwhile, make the fillings: In a medium saucepan, heat the milk and vanilla bean to a boil over medium heat. Immediately turn off the heat and set aside to infuse for 10 to 15 minutes. In a bowl, whisk the egg yolks and granulated sugar until light and fluffy. Add the cornstarch and whisk vigorously until no lumps remain. Whisk in 1/4 cup of the hot milk mixture until incorporated. Whisk in the remaining hot milk mixture, reserving the empty saucepan.
Pour the mixture through a strainer back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened and slowly boiling. Remove from the heat and stir in the butter. Pour into a bowl and cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming. Chill at least 2 hours or until ready to serve. (The custard can be made up to 24 hours in advance. Refrigerate until 1 hour before using.) To make the chocolate piping ganache, heat the cream to boil it and pour in over the chopped chocolate, whisking to blend. Let sit until thick enough to pipe. Place in a pastry bag with a small plain tip.
Using a long serrated knife, cut the eclairs partly open starting at one end and cutting halfway through to the middle of the eclair. Using a clean pastry bag, pipe the custard filling onto the eclair starting to fill in the uncut end then moving down to the cut end.
With the piping ganache, pipe a top and bottom row of teeth along the cut edge. Pipe a strip of green frosting on the top to make his back. With the chocolate ganache, pipe 2 eyes on the top of the eclair, 2 nostrils, and some wiggle-y, scaly looking lines like eyebrows over the eyes.