Combine the butter, water, milk, salt, and granulated sugar together in a medium saucepan over medium heat. Stir until the butter has melted. Bring mixture to a simmer. Once simmering, reduce heat to low and add the flour all at once. Stir until the flour is completely incorporated and the dough creates a ball. Move the dough ball around the pot for 1 minute, which gently cooks the flour. Remove from heat and transfer to the bowl of a stand mixer fitted with a paddle attachment or, if using a handheld mixer, a large mixing bowl. Allow to cool down for a few minutes before adding the eggs in the next step.
With the mixer running on low speed, slowly add the eggs in 3-4 separate additions mixing for 30 seconds between each. The mixture will look curdled at first, but will begin to come together as the mixer runs. Pour in the final addition of beaten eggs very slowly. Stop adding when the choux pastry has reached the desired texture: shiny, thick, and smooth with a pipeable consistency. Keep a couple teaspoons of the egg behind to use with your egg wash. Et voila, your choux pastry dough is complete!
Preheat oven to 400°F. Line two baking sheets with parchment paper or silicon baking mats. Lightly brush the parchment with water, which creates a humid environment for the pastry shells allowing them to puff up without drying out or burning. Transfer choux pastry dough to a piping bag fitted with a round piping tip. Pipe 2-inch mounds about 3 inches apart. Using a water moistened finger, smooth down the peaks and lightly brush each with egg wash. Bake for 20 minutes then, keeping the pastries in the oven, reduce oven to 350°F and continue to bake for 10 more minutes until golden brown. Keep an eye on them but do not open the oven as the pastries cook, as cool air will prevent them from properly puffing up. Remove from the oven and transfer to a cooling rack. Allow to cool completely before filling.
Cookie Butter Mousse Filling:
Using a handheld or stand mixer fitted with a whisk attachment, beat heavy cream on medium-high speed until soft peaks form, about 3 minutes. Temporarily transfer to another mixing bowl. Using a handheld or stand mixer fitted with a whisk attachment, beat the cream cheese on medium-high speed until creamy and smooth. Add the powdered sugar, vanilla extract, and cookie butter then beat on medium-high speed until combined and creamy. Using a rubber spatula, fold in the whipped cream until combined. Taste. Transfer mousse to a piping bag fitted with a small round tip or a filling piping tip.
Use a toothpick to prick a hole in 1 end of each eclair shell or if you're using a filling tip, poke it straight through. Carefully pipe mousse into each eclair shell, stopping when it feels full.
Warm up the cookie butter in a microwave safe bowl for 30 seconds, and then 15 second increments until soft and smooth. Dip the top of each into the cookie butter and place on a serving plate or cooling rack and sprinkle with crushed Lotus Cookies.