To make white chocolate pistachio mousse filling, in a food processor, pulse the pistachios until coarse.
In a saucepan, heat milk and pulverized pistachios over medium heat until scalding. Remove from heat, cover and steep for an hour.
Then strain the milk. Discard nuts. (<- or save it for your oatmeal.) Add more milk to get back to 1 cup, if needed.
In a small bowl, mix gelatin powder with 1 tablespoon of water. Let it bloom for 5 minutes.
Pour the milk into a saucepan, add gelatin and heat it over medium heat until milk is scalding and gelatin is dissolved, stirring frequently.
Meanwhile, beat egg yolks with sugar and salt until pale yellow.
To temper the egg yolks, take a cup of scalding milk and pour it into egg yolk while whisking continuously. Put the tempered egg yolk mixture back into the saucepan. Cook the sauce till it’s 180°F (82°C) stirring frequently. It shouldn’t come to a boil. Gently simmer over low heat. To test the doneness, tilt the saucepan, run the spatula on the bottom. If it leaves a clean trail, then it’s ready.
Place chopped white chocolate into a tall mug or bowl.
Pour the sauce over white chocolate. Let it sit for a minute to soften the chocolate. Now pour off half of the sauce into another bowl, leaving only chocolate portion, but don’t discard it.
Using an immersion blender, blend the chocolate mixture. As soon as the chocolate is fully melted and smooth, pour in the reserved milk mixture back into the chocolate mixture. Blend for a minute.
Add pistachio paste and blend again until smooth. Cool the mixture to room temperature.
In a mixing bowl with whisk attachment, add heavy cream and whip until hard peaks form. Fold the whipped cream into the pastry cream and mix until well combined.
Transfer the filling into a pastry bag with a small round tip and fill the pastry shells.
To make eclair shells, preheat oven to 375°F (190°C). Line 2 baking sheets with parchment paper, or silicone mat.
In a medium saucepan, combine water, butter, sugar and salt. Slowly heat the mixture over medium heat, stirring frequently to dissolve sugar and melt the butter. Once sugar is dissolved and butter is melted, increase the heat to medium high heat and bring the mixture to a boil.
When the mixture is boiling, remove the saucepan from heat and add flour all at once. Quickly mix the mixture until all the flour is absorbed. (We just made panade!)
Replace the saucepan back to burner and reduce the heat to medium. Cook the dough, continuously mixing with a wooden spoon. The dough will start to form into a ball. Keep mixing for another 3-4 minutes until light crust forms on the bottom of the pan. (Note: It’s important to cook for at least 3-4 minutes to dry out the panade.)
Transfer the dough into a mixing bowl with paddle attachment and mix it for a few minutes on low speed until the mixture is at room temperature, about 2 minutes. (No mixer? No problem. You can mix it with a wooden spoon!)
Add eggs one at a time, beating well after each addition. This will help to get egg absorbed into the dough quicker. Don’t add more than 2 eggs at a time! This is important, as you might not need to add all the eggs.
The dough should be smooth and shiny, thin enough to slowly fall into a ribbon, but too runny. (Read more details in this post.
At this point choux pastry is done and ready to be piped into whatever size you want. Uncooked dough can also be refrigerated for up to 3 days in an airtight container.
To bake pastry shells, transfer the batter into a large pastry bag and pipe into 4-inch strips or 2-inch rounds 3 inches apart. Level peaked tops with a wet fingertip.
Bake for 30-35 minutes, or until the shells are puffed up and golden brown, one baking sheet at a time. (Note: Don’t open the oven door the first 25 minutes.)
After initial 30 minutes, quickly open the oven door and make a small slit on the side of shells, using a small sharp pairing knife. This allows the steam to escape and helps to dry out the inside of pastry shells. Bake for another 5 minutes, or until shells are crisp and golden brown.
Transfer the shells onto a wire rack and cool completely before filling.
To make the white chocolate glaze, place chopped chocolate in a microwave safe bowl and melt it in a microwave according to package directions. (I microwaved mine at 50% power for 1 minute, and then 30-second increments at 50% power until fully melted, stirring after each stop.)
Add heavy cream and microwave one last time for 30 seconds at 50% power. Stir well until smooth.
To assemble eclairs, using a sharp pairing knife, make two holes on the bottom of each eclair shell and fill the pastry shells through the holes until the eclairs are filled. And then dip each filled eclair in the white chocolate glaze. Chocolate glaze will set in about an hour at room temperature. If not serving right away, refrigerate the filled eclairs.