To make the pastry cream, warm the milk in a medium saucepan over low heat, until bubbles form around the edges.

In a medium bowl, whisk together the sugar, flour, and salt. Add the egg yolks and whisk until well combined. It'll be thick, and a little hard to stir, but that's okay. Whisk in a bit of the warmed milk into the dry ingredients. Once combined, continue to whisk in the warmed milk until all of it has been added. Pour the mixture back into the saucepan and set over medium heat. Continue to whisk and cook until thickened, about 5 to 8 minutes. The mixture will be very thin at first but continue to cook and whisk until it's thickened like a pudding. Don't stop whisking, as the mixture might stick if you do. Once thickened, let the mixture come to a boil and cook for about 30 seconds. Remove from heat, stir in the butter and vanilla until well combined and then pour into a bowl, through a fine mesh sieve/strainer. Cover with plastic wrap, making sure the plastic wrap touches the surface of the cream. Chill until completely cold, about 4 hours or overnight.

Preheat oven to 425 degrees F. Line 2 baking sheets with parchment paper and set aside.

To make the eclairs, in a medium saucepan bring the water, butter, salt and sugar to a rapid simmer. Remove from heat and immediately stir in the flour with a wooden spoon. Keep stirring rapidly until the mixture comes together for about 30 to 60 seconds. Return to the heat and cook, constantly stirring rapidly, for about 30 seconds. Transfer the mixture to a stand mixer, fitted with the paddle attachment, and stir on medium. Slowly add the eggs, one at a time, mixing well after each addition. Scrape down the sides and bottom of the bowl and mix once more. The batter should be thick, but fall slowly away from the paddle. Fold in the sprinkles. Transfer to a large pastry bag fitted with a plain circle tip.

Pipe out the eclairs onto the prepared baking pans, to about 2 inch think logs. Make sure to leave about 2 inches of space apart. Brush the surface of each eclair with the egg wash and bake for 15 minutes. Reduce the heat to 375 degrees F and continue to bake until puffed and golden brown, about 25 minutes more. Let the eclairs cool completely on the baking sheets before filling.

Using a small sharp pairing knife, make a small incision on both sides of each eclair, this will make them easier to fill. Transfer the chilled pastry cream into a piping bag with a small plain circle tip. Fill the eclairs with the cream and return to the baking sheets.

Pour the candy melts into a medium bowl and microwave in 30 second intervals until melted through and smooth. Pour the sprinkles into a separate dish. Dip the tops of each eclair into the candy melts and then into the sprinkles making sure they're well covered. Return to the baking sheet and let set before serving. Serve immediately or keep in refrigerator until ready to eat. They can be kept in the fridge for 4 days. Enjoy!