Coffee Cream Puffs (Choux au Craquelin)

Coffee Pastry Cream
1 Make the pastry cream first or even the day ahead, so it can chill in the fridge until ready to fill the cream puffs. 
2 In a large saucepan on medium heat, stir together the milk, espresso powder, vanilla, and half of the sugar (60 grams). 
3 Meanwhile, using a separate mixing bowl next to the stovetop, whisk together the egg yolks and second half of the brown sugar until creamy. 
4 Sift the flour and cornstarch into the egg yolk mixture and whisk well until thick and foamy.
5 Once the milk is steaming, slowly pour it into the egg mixture while whisking vigorously to temper the eggs. Then pour everything back into the saucepan and return to medium heat.
6 Continue to whisk to prevent lumps as the pastry cream thickens. As it thickens, it will come to a boil. You'll notice it's ready when you stop whisking and the mixture burps up a few air pockets.
7 Whisk vigorously to achieve a smooth pastry cream, then remove from the heat. Transfer the pastry cream to a heatproof bowl and whisk in the softened butter until glossy.
8 Place a sheet of plastic film directly on the surface and refrigerate until cool.
Brown Sugar Craquelin Disc
10 Mix softened butter and brown sugar into a paste in a medium-sized bowl using a silicone spatula. Add in the flour and mix until you have a soft cookie dough.
11 Gently pack the dough into a disc with your hands. Place the dough between two sheets of parchment paper. Roll into 1/8-inch thickness. Transfer to the freezer for 10 minutes. 
12 Using a small cookie cutter, cut out 14 discs. Re-roll scraps if needed. 
13 Freeze the discs in one layer between parchment paper sheets until ready to bake.
Choux Pastry
15 Instead of making a bunch of pastry cream for a full batch of choux pastry, I'm using a half recipe for 12-14 cream puffs.
16 In a medium saucepan on medium heat, combine the milk, water, butter, sugar, and salt. Once the butter is melted, give it a little stir and continue heating until it reaches a boil.
17 Remove the saucepan from the heat and dump in all the sifted flour at once. Immediately begin mixing with a silicone spatula to prevent any lumps from forming.
18 Return the pan to medium-low heat and continue mashing and spreading the dough with a spatula to cook out some of the moisture. The dough is ready when it forms a cohesive ball as you shake the pan and a thin film forms on the bottom of the pan. About 2-3 minutes.
19 Transfer the dough to a separate mixing bowl and stir until you no longer see any steam. Mix in a quarter of the beaten eggs at a time, making sure each addition is thoroughly incorporated before adding the next.
20 The final texture should be pipeable, neither too thick nor too runny. It will be smooth and slightly glossy. If you don't use all of the eggs or find you need a touch more, that's ok.
21 After mixing in the eggs, lift the spatula to see the dough slowly hanging in a V-shape. If you drag a spatula through the dough, it should leave a distinct path that slowly fills back in, rather than staying firmly open or rushing back together.
26 Transfer batter to a plastic piping bag fitted with an 11mm round tip.
Pipe and Bake
27 Preheat conventional oven to 375℉ (180℃). 
24 Line a baking sheet with either parchment paper or a silicone baking mat. Draw or mark circles on the paper that are 1.5 inches in diameter, spaced 3 inches apart.
25 Hold the piping tip perpendicular to the pan, a finger's thickness above the surface, and pipe 12 rounds of choux. Use pressure from the top of the piping bag to build each mound, then release the pressure while moving the tip in a small circular motion to finish. Don’t worry if they have little tips; you’ll cover them with craquelin.
28 Gently place a frozen craquelin disc on the center of each choux mound. 
29 Place the pan on the middle oven rack and bake for 28-30 minutes. The choux pastry will puff into rounded mounds, and the craquelin will turn a nice dark golden brown with a broken, crispy texture. DO NOT open the oven until they're finished baking, or they may collapse.
30 Place the tray on a wire rack to cool for 15 minutes. Then gently transfer the cream puffs onto the wire rack to finish cooling.
Fill with Pastry Cream
31 Pull cold pastry cream from the fridge. Whisk vigorously by hand to loosen and smooth it back to pipeable consistency.
32 Transfer to a plastic piping bag fitted with an 8mm round piping tip.
33 Cut a small hole in the bottom of each cooled cream puff. Insert the pastry cream tip slightly, then fill with pastry cream until there is a little resistance. Scrape any pastry cream that may have oozed out from the bottom. Re-invert right side up and serve.