Preheat the oven to 425ºF.
To make the puffs, melt the butter in the water with salt and sugar. Remove from heat, add the flour, return to heat and beat vigorously for 2 to 3 minutes. (A film should form on the bottom of the pan.) Cool slightly, and add eggs, one at a time, beating vigorously.
Using a pastry tube with a 1/2-inch opening, form 1-inch-high mounds, 3/4-inch in diameter, on parchment lined baking sheet. Glaze and smooth the tops. Bake for 20-25 minutes, until puffed and golden. Cool on racks.
To make the mocha cream, beat the egg yolks, gradually adding the sugar, until mixture is thick and pale yellow. Beat in the flour. Scald the milk and add in dribbles, reserving 1/2 cup for thinning. Return to clean pot and stir vigorously over high heat until mixture boils and thickens. If it seems too thick to pipe, add reserved milk. Remove from heat.
Add the butter, 1 tablespoon at a time. Melt the chocolate and add to mixture with the espresso. Just before assembling croquembouche, inject the cream into the puffs with a 1/4-inch pastry tip.
To make the caramel, bring sugar, water, and corn syrup to a boil over high heat. Do not stir. Cover pan until steam dissolves any crystals. Uncover and boil 5 minutes, or until syrup is amber. Remove from heat. Dip the bottoms of the puffs, one by one, into the caramel and arrange in a pyramid. Cut the tip from a balloon whisk, dip into caramel, and whirl strands of caramel around croquembouche to form a spun-sugar web.
