Line a baking sheet with parchment., In a medium bowl, whisk together the flour, cornmeal, sugar, baking powder, and salt., Work in the butter until the mixture resembles coarse meal., Add the lemon zest, juice, eggs, and 1/4 cup of the milk, mixing until the dough is cohesive. If the mixture is still dry add more milk, a tablespoon at a time, until it comes together., Turn the dough out onto a lightly floured piece of parchment and use the parchment to fold the dough in thirds. Pat or roll the dough about 1" thick. Freeze the dough for 20 minutes while you preheat the oven to 400°F., Remove the dough from the freezer and cut it into rounds with a 2 1/2" biscuit cutter. Gather, stack, and recut the scraps as needed., Brush the tops of the biscuits with milk and sprinkle with the sparkling sugar., Bake the biscuits or 20 to 22 minutes, until the tops and edges are golden brown. Remove them from the oven and serve warm., Store biscuits, well wrapped, at room temperature for two days; freeze for up to one month.