Preheat oven to 375 degrees F (190 degrees C).
Into a pot, add Salted Butter (1/2 cup) and Water (1 cup). Bring to a light boil.
Add All-Purpose Flour (1 cup). Lower heat to medium. Cook for 2-3 minutes, stirring constantly.
Into the bowl of a stand mixer fitted with a paddle attachment, add the batter. Mix on medium speed for 1 minute.
Add Egg (4) one at a time, beating for about 30 seconds in between each.
Scoop 2 tablespoons of the batter out at a time. Space them evenly across a baking tray lined with parchment paper. You should be able to make about 20 balls.
Into a small mixing bowl, add Egg (1) and Milk (1 teaspoon). Beat until well-combined. Brush the dough balls with the egg wash.
Bake for 25-30 minutes. Do not open the oven early!
Into the bowl of a stand mixer, add Peanut Butter (1 1/2 cup), Milk (1/2 cup), and Vanilla Extract (1 tablespoon). Mix to combine.
Add Powdered Confectioners Sugar (1 1/2 cup). Mix until well-combined, adding more milk if needed. Set in the fridge to chill until ready to use.
Into a saucepan over medium heat, add Heavy Cream (1/4 cup) and bring to a near-boil.
Pour cream over Dark Chocolate (4 ounce). Let sit for a few seconds, then mix until melted and smooth.
Cut each profiterole in half. Place a dollop of the peanut butter mixture in the middle, then press the two sides back together. Drizzle with chocolate ganache.