For the crust: Pour the flour, cornmeal, sugars and salt into the bowl of a food processor fitted with a metal blade and process to combine. Scatter the butter over the top and pulse with the dry mixture to coat, and then continue pulsing until cool and almost sandy, about eight 3-second pulses. Drizzle the yolk over the top, and pulse again until the ingredients are moist and barely starting to clump, about another 8 pulses. The dough should just hold together when you squeeze it in your palm. If not, add 1 teaspoon of cold water and pulse briefly.
Turn the dough out onto a lightly-floured work surface. Smear out the dough across the work surface using the heel of your hand until it begins to come together. Gather the dough back together and shape it into a disc. Wrap in plastic and chill for 1 hour. Let the dough warm up slightly before rolling it out (if you roll it while it is cold, it will crack). Roll the dough on a lightly-floured work surface with a rolling pin into a 1/8 to 3/16-inch-thick circles, 10 to 11 inches in diameter. Fit the dough into an 8-inch tart pan with a removable bottom. Don't worry if it cracks a bit; you can patch it with some of the trimmings. Press the dough firmly into the bottom of the pan where it meets the sides, and then roll the rolling pin across the pan to cut off the overhang. Press the dough firmly against the sides of the pan and back up to the top using your fingers (sometimes the rolling pin pushes the dough down the sides of the pan a bit). Chill in the refrigerator for 30 minutes.
Preheat the oven to 350 degrees F. Position the rack on the lowest rung of the oven and put a baking sheet on top. Line the chilled crust with a 10-inch coffee filter and weight it down with dried beans or rice. Place on the hot baking sheet and bake until the sides begin to turn golden brown, 15 to 20 minutes. Remove the shell from the oven, carefully remove the beans and filter and then return to the oven until completely golden brown and baked through, another 10 to 15 minutes. Reduce the heat to 325 degrees F. Let cool before adding the filling, about 10 minutes.
For the filling: Pulse the maple sugar, flour and salt in a food processor until combined. Add the melted butter and pulse to combine, and then add the buttermilk and vanilla and pulse to combine again. Add the eggs and pulse until just combined-don't over-mix. Strain into the tart shell. Bake the tart until the sides are set (they may puff slightly) and the middle still jiggles slightly when you tap the pan, 30 to 35 minutes. Cool on a rack for at least 45 minutes. Serve warm or at room temperature, garnished with cookies as desired.
You can roll and bake the scraps from the trimmed tart shell to make little cookies for garnishing the finished tart. Sprinkle them with sanding sugar and bake them at the same time you blind bake the crust; watch to make sure they don't over-bake. You can also pop them in the oven at 325 degrees F for 10 minutes while the baked tart cools. If the filling curdles during the pie-making process, don't worry. It will come out fine.