For the choux:
Recipe from Williams Sonoma. Yields about two dozen mini batons.
1/2 cup whole milk
1/2 cup water
6 Tbs. (3/4 stick) unsalted butter, cut into 1/2-inch pieces
1/4 tsp. salt
1 cup all-purpose flour
For the lemon pastry cream:
1 cup whole milk
1 tablespoon lemon zest
4 egg yolks
1/2 cup granulated sugar
3 tablespoons all-purpose flour
1 tablespoon butter
1/4 teaspoon vanilla extract
3 tablespoons fresh lemon juice
For the blueberry white chocolate ganache:
1 cup fresh blueberries, plus extra for decorating
1/4 cup sugar
1/3 cup heavy cream
6 ounces white chocolate
First, make the choux. Preheat the oven to 425°F. Line two sheet pans with parchment.
In a saucepan over medium-high heat, combine the milk, water, butter and salt and bring to a full boil. When the butter melts, remove the pan from the heat, add the flour all at once and stir vigorously with a wooden spoon until blended. Return the pan to medium heat and continue stirring until the mixture pulls away from the sides of the pan and forms a ball. Remove from the heat and let cool for 3 to 4 minutes, or to 140°F on an instant-read thermometer.
Meanwhile, in a small bowl, whisk 1 of the eggs. When the batter has cooled, add the egg and beat with the spoon until incorporated. Whisk each of the remaining eggs one at a time, then stir into the batter. After each egg is added, the mixture will separate and appear shiny but will return to a smooth paste with vigorous beating. Let the paste cool for about 10 minutes before shaping.
To shape mini batons, fit a pastry bag with a 1/2-inch plain tip and fill the bag with the paste. Pipe out logs 3 inches long and 3/4 inch wide. Space the logs at least 2 inches apart.
Bake the batons for 15 minutes, then reduce the heat to 375°F and continue baking until golden brown, 10 to 15 minutes more.
Remove from the oven and immediately prick the side of each baton with the tip of a sharp knife. Return to the oven, leave the oven door open and allow the pastries to dry out for 10 to 15 minutes. Transfer the pans to wire racks and let the pastries cool completely on the pans before filling.
For the lemon pastry cream, start by bringing the milk and lemon zest to a simmer in a medium saucepan over medium-low heat. Remove the pan from the heat and allow the mixture to cool for 5 minutes.
In the meantime, combine the egg yolks, sugar and flour in a medium bowl. Once the milk mixture has come to a simmer, slowly pour a small amount into the yolk mixture, whisking continuously. Continue tempering the yolks with the milk mixture. Then, return the mixture to the saucepan and whisk constantly over medium-low heat until smooth and thick. Remove the pan from the heat and stir in the butter. Whisk in the vanilla and lemon juice. Transfer the pastry cream to a clean bowl and lay a piece of plastic wrap directly onto the surface. Refrigerate for at least 2 hours.
Once it has cooled, transfer to a piping bag with a very small round tip.
For the blueberry white chocolate ganache, combine the blueberries and sugar in a saucepan and set over medium heat, stirring occasionally until the blueberries begin to burst and the mixture is a syrup-like consistency. Transfer the blueberry mixture to a food processor and process until smooth. Strain through a fine-mesh sieve into a bowl.
Strain the blueberries through a fine mesh strainer. You don't want to have the skins of the berries in the ganache, just the juices. Set aside to cool.
Roughly chop the white chocolate and place in a medium bowl. In a small saucepan, bring the heavy cream to a simmer. Once the cream has reached a simmer, pour over the white chocolate pieces and immediately cover the bowl with plastic wrap. Let stand for about 10 minutes to allow the cream to melt the chocolate. Remove the plastic wrap and gently whisk until smooth. Add 3 to 4 tablespoons of the blueberry syrup to taste. Cover with plastic wrap and refrigerate until cool.
Once the ganache is firm, using a hand mixer whip the ganache for a few minutes until it increases in volume and is lighter in color. The whipped ganache can be stored in the refrigerator until it is ready to be used.
To fill the éclairs, gently use a toothpick to create a small hole on each end of the éclairs. Using a pastry bag fitted with a medium-size plain tip, gently pipe the custard into either end of the eclairs, using only just enough to fill the inside Repeat this process until all of the éclairs are filled.
Using a small offset spatula or butter knife, spread a dollop of the ganache onto each éclair. To decorate the éclairs, roll just-washed blueberries in sugar. Top each éclair with a sugared blueberry. Serve immediately.
Make ahead tip: The choux can be made in advance and kept in the freezer for up to one month until ready to thaw and be filled. The pastry cream and ganache can be made up to a day in advance and kept in the refrigerator until time to fill and decorate the pastries.