Make the churro donuts: Place the water, milk, butter, sugar, and salt in a medium saucepan and bring to a boil over medium-high heat. Once boiling, remove the pan from the heat and add the flour, stirring with a wooden spoon until the flour is completely incorporated.
120 g milk, 120 g water, 90 g unsalted butter, 15 g granulated sugar, ½ tsp fine sea salt, 125 g all-purpose flour
Place the pan back over medium heat and continue to cook and stir for 1 to 2 minutes to steam away as much water as possible. (The more moisture you can remove, the more eggs you can add and the lighter your pastry will be.) The mixture is ready when a thin film coats the bottom of the pan.
Move the mixture to the bowl of a stand mixer fitted with the paddle attachment and mix for about 1 to 2 minutes to allow it to cool slightly. Add the first egg and mix on medium speed until completely combined, scraping down the sides of the bowl as needed.
4 large eggs (200g)
Add the remaining eggs, one at a time, and mix in completely until the batter becomes smooth and glossy. If the consistency is right, when you pull the paddle up the dough sticking to the paddle should hang down in a V shape ribbon that slowly breaks off. If your batter isn't the right consistency, add some of the additional whisked egg. Once you achieve the right consistency, transfer the batter to a piping bag fitted with a large open star tip.
Using a cookie cutter or other circular tool, draw 6 circles 3-inches in diameter on a piece of parchment paper. Place the parchment paper upside down on a lined baking sheet and pipe out 6 circles. Top with the desired amount of chocolate chips and then pipe another circle of batter over the top of the chocolate. Keep in mind you need enough batter to cover all of the chocolate; make sure there are no gaps in between each layer otherwise, the melted chocolate will leak out & you’ll make a mess when frying.
Immediately transfer the tray to the freezer and freeze until solid, about 2 hours or overnight.
In a small bowl, whisk together the sugar and cinnamon until combined. Set aside.
100 g granulated sugar, 1 tbsp ground cinnamon
Fry the churro donuts: Heat at least 2 inches of oil in a heavy-bottomed pot until a thermometer registers 360-370°F (180-190°C). Working in batches, place two donuts at a time into the oil. Fry on each side until golden brown, about 3 to 4 minutes on each side. Remove fried donuts with tongs and drain on a paper towel, then toss in cinnamon-sugar mixture. Repeat with the rest of the piped churro donuts.
Make the chocolate dipping sauce: In a small saucepan, whisk together the cocoa powder, granulated sugar, espresso powder, and salt until no lumps remain. Add the water, and bring to a boil over medium-high heat.
20 g unsweetened natural or Dutch process cocoa powder, 100 g granulated sugar, ⅛ tsp espresso powder, ⅛ tsp fine sea salt, 60 g water
Once boiling, reduce the heat to medium-low and allow to simmer until thickened, about 1 minute. Remove from the heat and stir in the vanilla. The chocolate sauce will thicken as it cools.
½ tsp vanilla extract