Preheat oven to 400°F. Line 2 large cookie sheets with parchment paper.
In a medium saucepan over medium heat, mix the butter, water, milk, sugar, and salt until everything is melted. Turn heat to low and whisk in flour, stirring for 3 minutes. Remove from heat and place into bowl of a standing mixer.
Using a whisk attachment, begin to whisk flour mixture. Slowly and one at a time, add in eggs, beating in between each addition. Once everything is mixed in, transfer to a piping bag fitted with a 1/2 inch tip.
Pipe 1 1/2 inch rounds onto prepared cookie sheets. You should have roughly 24 rounds per cookie sheet. Bake for 25 minutes, until the puffs are perfectly golden brown. Allow to cool completely before piping.
To make the pastry cream
In a medium saucepan over low heat, heat the milk, coffee, espresso powder, and 1/4 cup of the sugar until scalding.
In a separate small bowl, whisk the egg yolks, other 1/4 cup of sugar, and cornstarch. Pour roughly 1/4 cup of the hot milk mixture into the eggs, whisking the entire time. Pour the warmed egg mixture into the saucepan. Whisk constantly until the mixture thickens into a custard, roughly the texture of yogurt, about 15 minutes. Immediately remove from heat and place in an ice bath to stop cooking. Allow to cool completely.
To make the croquembouche
Place the mocha custard into a large piping bag with a small tip. Use the piping bag to pop a small hole in the bottom of each profiterole, then pipe the custard into the inside of the profiterole.
Heat the sugar and salt in a small saucepan over low heat until the sugar dissolves into liquid and becomes a light amber hue. Remove from heat and place in ice bath to stop cooking.
Very carefully, dip the tops of each profiterole into the sugar, then place on cookie rack to cool slightly. Top each profiterole with a pinch of coffee granules (optional).
You are now going to make the croquembouche. You will make roughly 6 layers of profiterole circles, beginning with the bottom layer consisting of roughly 12 profiteroles and creating a hollow pyramid from there. Dip each profiterole one at a time into the hot sugar mixture, this time on two sides at 90° angles to each other. Place profiterole onto a cake plate on its side with top facing outward, beginning your biggest circle. Continue to dip each profiterole into the sugar on two sides, placing one side down on the cake plate and attaching the other side to the profiterole before it, until you create a profiterole circle.
Continue the process in layers, making sure that each layer hardens before adding the next. If the sugar gets too sticky, place over low heat to warm it up again.
Once built, use a large spoon to swirl remaining sugar around and around the croquembouche.
Heat the chocolate, coffee, and coconut oil in a small bowl until completely melted, then drizzle over croquembouche. Top everything with flaked sea salt.