In a medium saucepan, combine the water, milk, butter, sugar, and salt, bringing to a boil over medium heat.

Reduce the heat to low and add the flour, stirring constantly until the dough forms a ball.

Transfer to the bowl of your stand mixer and mix on low speed for 3 minutes to cool.

Add eggs one at a time, thoroughly incorporating each before adding the next.

Cover with plastic wrap and chill for at least a half hour (and up to 24 hours) before using.

In a medium saucepan, combine the milk and half of the sugar over medium heat.

In a small bowl, whisk together the egg yolks, other half of the sugar, cornstarch, and vanilla extract.

Once the milk is gently boiling, remove from heat and slowly pour a ribbon of about 1/3 of the hot milk into the egg yolk mixture, constantly beating with the whisk to ensure proper tempering of the eggs.

Pour the eggs into the rest of the hot milk and bring to a boil over medium heat, beating constantly with the whisk.

Once the mixture has thickened, cook it for another 3 minutes to ensure the starch is cooked off.

Transfer to a clean bowl and cover with plastic wrap over the surface of the pastry cream to ensure no skin forms.

Keep in refrigerator until cooled completely and up to 2 days before using.

When ready to bake the eclairs, preheat oven to 375 degrees Fahrenheit and line 2 baking sheets with parchment paper.

Scoop the dough into a pastry bag (or I used a Pampered Chef Easy Accent Decorator) fitted with a large star tip.

Pipe out lines of eclairs, spacing 1 inch apart on the sheet. You can add layers on top of layers to make them puffier--in fact, I recommend it if they're a little on the thin side initially.

Brush the tops of each line of dough with the beaten egg.

Bake both sheets for 25 minutes, rotating halfway through, or until golden on top.

Transfer to a wire rack to cool completely.

In a microwave safe bowl, heat the chocolate at 50% power for 30 second intervals, stirring in between each, until the chocolate is completely melted.

Stir in the cream and butter until smooth.

Transfer the pastry cream to a pastry bag or Easy Accent Decorator, fitted with the long, narrow tip for filling.

Gently push the tip into one end of an eclair and fill, then repeat on the other end.

Dip filled eclair on the eggwash side in the chocolate.

Assemble on a wax paper lined tray and transfer to the refrigerator for 30 minutes.

Serve or transfer to an air-tight container for longer storage, up to 2 days.