Fill a large bowl with ice, and enough water to fill the bowl halfway. Place a medium bowl in it with a fine mesh strainer nearby along with a piece of buttered wax paper. Set aside.
Place the milk in a medium saucepan and heat over medium low heat until just before it begins to boil.
While the milk heats, place the sugar and egg yolks in a medium bowl and whisk until well incorporated. Add the flour and whisk vigorously until smooth. When the milk is hot enough, add a few tablespoons of it to the egg mixture while whisking constantly to temper the eggs. Add 1/3 of the mixture in a slow steady stream while whisking constantly. Once 1/3 has been added, add the rest all at once while whisking constantly until combined.
Place the mixture back in the saucepan and cook over medium heat, whisking constantly to prevent lumps, until the mixture begins to boil, about 8-10 minutes. Continue to cook for another minute until very thick. Remove from heat and whisk in the vanilla and softened butter, then place in the strainer over the bowl set over the ice bath and strain to remove any lumps or bits of egg. Cover the pastry cream with the buttered wax paper to prevent a skin from forming and allow to cool at room temperature. Once cooled, remove from the ice bath and place in the refrigerator until ready to use. For best results, make in advance and store in the fridge for 4-6 hours or overnight.
Prepare the ganache:
Place the chocolate chips, heavy cream, and corn syrup in a heat proof bowl. Set aside.
Fill a large sauté pan halfway full of water and heat over medium heat until the water simmers. Turn the heat down to low, then place the bowl of chocolate into the water in the sauté pan, stirring constantly with a rubber scraper until the chocolate melts and emulsifies with the cream. When the mixture is smooth, remove from heat and set aside to cool. May be made in advance and reheated until it reaches the desired consistency (a thinner consistency is best for dipping).
(1) batch of Pate a choux, recipe link in Notes.
Assemble the eclairs:
Place the pastry cream in a piping bag fitted with a round tip if inserting the cream into the eclairs, or cut the eclairs in half lengthwise and pipe into the bottom of the eclair with a decorative tip (I used Wilton 1M).
Heat the chocolate ganache to thin it out, if necessary, and dip the tops of the eclairs into the chocolate, gently shaking off any excess chocolate. Place on a baking sheet, sprinkle with the funfetti sprinkles, and set aside to set briefly. Repeat with all of the eclairs.
Serve and enjoy!
Notes
Cook time and prep time includes making the pate a choux, pastry cream, and ganache. It does not include chilling time for the pastry cream.
*I added corn syrup to the ganache, which makes it nice and glossy. It can easily be omitted if you prefer not to use it.
Eclairs are best assembled and eaten the day they're made, but will keep up to 2 days in an airtight container in the fridge.