Grease the pie pan, place it upside down on top of a piece of paper, and trace the circumference of the pan. Then nudge the pan a few inches to the side to reveal a crescent shape in the tracing. Trace the edge of the pan to create the crescent shape. Place the pan right side up again and set it aside.
In a large bowl, mix together the flour, sugar, and salt until combined. Using a box grater, grate the butter using the large hole setting of the grater above the bowl, stopping to stir and coat the butter bits in the flour mixture every 10 seconds or so. When all the butter is in, pinch the mixture together with your fingertips until it resembles the texture of damp sand. Add the water, a tablespoon at a time, mixing it with a fork, until the dough just holds together when you squeeze a fistful of it in your hand.
Divide the dough into two portions, one slightly larger than the other (about 1/3 vs 2/3). Roll out the larger quantity of dough until it is 1 cm thick and transfer it to the greased pie pan. Press the crust into the pan, trim off the excess leaving a 1-inch overhang. Fold the 1-inch overhang under the edge of the crust all the way around and pinch the edges. Cover and place the pan in the freezer for 30 minutes.
Roll out the smaller dough until it's 1 cm thick. Cut out the traced crescent and place it over the dough. Use a pizza cutter, pastry cutter, or sharp paring knife to cut out the crescent shape. Discard the paper. Use cookie cutters or pie cutters to cut out more decorative shapes, or you can cut them out of the dough by hand with a knife.
In a small bowl, whisk together the egg and milk to create the egg wash. Place a piece of parchment paper on a small baking sheet and place the crescent on top of it. Brush it lightly with the egg wash, and brush any decorative cut-outs with the egg wash. Arrange the decorative cut-outs on the crescent, and place the pan in the freezer for 45 minutes.
Preheat the oven to 350 degrees Fahrenheit. In the bowl of a stand mixer fitted with the paddle attachment, mix together the granulated sugar, brown sugar, cornmeal, and cocoa powder at low speed. Add the eggs, one at a time, and mix until fully incorporated. Then mix in the evaporated milk, butter, and vanilla extract until completely combined.
For the top decoration, place the baking sheet with the crescent on it in the oven and bake at 350 degrees until golden, about 30 minutes (it can be in the oven at the same time as the pie that follows). Allow the crescent to cool to room temperature.
Pour the filling into the prepared pie crust and bake on the second lowest rack of the oven until the filling is set around the edges and the center has a *very* small wiggle to it, about 1 hour to 1 hour 10 minutes. (If the crust is browning too quickly, cover the edges with tin foil but don't let the tin foil touch the filling). Remove from the oven and allow to cool for 30 minutes before sprinkling with the flake sea salt, garnishing the top with the crescent, slicing the pie, and serving.