Combine the water, salt, butter, and sugar in a saucepan and bring to a simmer over high heat. There's no need to stir.
Turn the heat down to medium and add the flour all at once. Stir with a wooden spoon for about 3 minutes, or until the mixture forms a cohesive mass and pulls away from the sides of the pans, leaving a film of flour on the sides of pan.
Transfer the mixture to a bowl or a stand mixer and beat it with a wooden spoon or the paddle attachment for about 1 minute or until slightly cooled.
Beat in the eggs, 1 at a time, until you've added 4. Check the consistency of the batter by pulling a wooden spoon through it. When the groove it makes slowly in on itself, you've added enough eggs. Another way to check is to see if the batter droops from the spoon when it is held sideways. If so it is ready. Add more egg whites as needed until the batter passes away the groove test. While the dough is best used right away, it will keep for several hours in the refrigerator.
Preheat the oven to 400F. Fit a pastry bag with a 1/2-inch plain tip or french star tip and fill it with the dough. Prepare a cookie sheet with parchment paper or silicone baking mat. If using paper, pipe a small amount of dough underneath the paper in the four corners to hold it down.
Make 2 rows of 3 eclairs each (if making minis I fit 3 rows). Start by holding the pastry bag about 3/4-inch above the sheet pan and squeezing. If the pastry bag tip is too close, the eclairs won't be thick enough; if it's too far away, they won't be wide enough. As the round of pastry takes shape, lift the pastry bag until it is about 1 1/2 inches above the sheet pan. The mounds of dough should be about 3 1/2 inches in diameter and about 1 1/2 inches high in the center. Brush with an egg wash, lightly smoothing the surface with the brush at the same time. Bake the cream puffs for about 20 minutes or until puffed and golden. Turn down the oven to 300F and continue baking for 20 minutes more. Let cool for at least 20 minutes on a cake rack.
Prepare the whipped cream by placing the heavy cream into a bowl of a stand mixer. Combine with the sugar and vanilla extract and whip to soft peaks. Use a serrated knife, cut each cream puff open horizontally. Pipe the whipped cream or scoop with an ice-cream scoop the whipped cream into the middle and top with sliced strawberries. Top with additional powdered sugar if you choose and I think you should!
For the pastry cream: Whisk egg yolks until smooth in a large bowl. Combine sugar, cornstarch, and salt in a medium saucepan, and heat over medium. Stirring constantly, gradually add milk in a slow, steady stream, and cook until mixture thickens and begins to bubble, about 5 minutes.
Whisking constantly, slowly pour one third of the milk mixture into egg yolks (this step is called tempering, which keeps the yolks from curdling). Pour mixture into remaining milk mixture in saucepan. Cook over medium heat, whisking constantly, until mixture comes to a full boil and is thick enough to hold its shape when lifted with a spoon, 2 to 4 minutes. Remove from heat; stir in vanilla.
Strain mixture through a fine sieve into a heatproof bowl. Cover with parchment paper or plastic wrap, pressing it directly on surface to prevent a skin from forming. Refrigerate until chilled and firm, at least 2 hours (or up to 2 days). When ready to use, fill a piping bag with a small round tip. Poke holes with a toothpick into the center (you can poke holes into the sides, bottom or top of the eclair. If you poke holes in the top, you are less likely to have filling ooze out). Then pipe the pastry cream into the holes. Stop piping when you see the cream start to come out the holes.
For the ganache: In a heavy saucepan, boil heavy cream (You can also heat the heavy cream in a microwave safe bowl). Turn off the heat. Add chopped chocolate pieces and let it rest until melted. Use a rubber spatula to stir the mixture until all the pieces are melted. Dip the top of eclairs into the ganache. Or drizzle over each eclair.