Make the graham crakcer craquelin. Pulse together the flour, brown sugar, graham cracker crumbs, salt, cinnamon, and butter in a food processor until it forms a dough, about 1 minute. Shape the dough into a disc and place it between two sheets of parchment paper or wax paper. Roll out the dough until it's ⅛-inch thick and chill in the fridge for 1 hour or in the freezer for 30 minutes. Cut out 2-inch circle discs and keep them in the fridge until ready to use.
Meanwhile, make the key lime curd. Whisk together the key lime juice and zest, sugar, and eggs and egg yolk in a heat-safe bowl. Place the bowl over a pot of simmering water, making sure the bowl doesn't directly touch the water. Whisking frequently, continue to cook the curd until it's thick enough to coat the back of a spoon, about 15 minutes. Remove from heat and whisk in the butter until it's fully incorporated. Place plastic wrap directly on top of the surface of the curd and let cool completely.
Preheat oven to 400 degrees F. Line 2 baking sheets with parchment paper.
Make the cream puffs. Combine the butter with water and salt in a heavy saucepot. Heat over high heat and bring to a rolling boil. Lower the heat to low and add the flour to the butter-water mixture. Beat with a wooden spoon until the dough comes together. Remove from heat and add the eggs one at a time, fully incorporating each egg before adding the next.
Transfer the choux pastry to a piping bag and pipe 16 cream puffs on the baking sheets (8 per sheet pan), giving them plenty of space in between. Place a graham cracker craquelin disc on top of each of the cream puffs. Bake the cream puffs for 20-25 minutes, rotating the sheet pan halfway through. The cream puffs are ready when both the tops and bottoms of the puffs are golden brown. Remove from the oven and let cool completely.
Meanwhile, make the whipped cream. Whip the heavy cream with powdered sugar until stiff peaks form. Transfer the mixture to a piping bag.
Cut the cream puffs in half. Place 1 heaping tablespoon of the key lime curd in the center and pipe the whipped cream around the curd. Sprinkle lime zest and place the top of cream puff on top. Serve immediately.