Adapted from Ivy Manning's Crackers & Dips
Preheat oven to 375 degrees. Line two baking sheets with parchment paper. In a food processor, combine the cornmeal, flour, baking powder, sugar, salt, and cayenne; pulse to combine.
Add the butter and pulse to cut it into the dry ingredients until the mixture resembles coarse cornmeal. Add the buttermilk and egg and pulse just until the dough comes together.
On a lightly floured surface, knead the dough until smooth, about 5 strokes. Divide the dough in half and form each half into a small rectangle that is about 1 inch thick. Wrap the dough in plastic wrap and chill for at least 30 minutes and up to 24 hours.
Place one piece of dough on a lightly floured surface and roll it out to 1/8 inch thick. Use a ring cutter to cut the dough into 1" rounds. With a lightly floured spatula, transfer the rounds to the baking sheets. Repeat the process with the scraps.
Bake until the crackers are golden brown on the bottom and dry to the touch, 15 minutes, rotating the baking sheets once from top to bottom and front to back.
Cool the crackers on a rack and store in an airtight container for up to 5 days.
Wash, trim, and slice the rhubarb thinly across the grain.
Wash the orange and lemon, then quarter them to remove any seeds. Roughly chop them, then grind in food processor until the rind is in pea-sized pieces or smaller.
Combine the sliced rhubarb, ground orange and lemon, and sugar in a large pot and simmer uncovered, stirring frequently, until the mixture is thick, like a jam. Taste and add more sugar or lemon juice if needed. If the citrus rind still has an astringent bite, add a bit more water and continue to cook until that taste has disappeared and the marmalade is the correct consistency.
Lay out the corn crackers on a baking sheet and spread a thin layer of ricotta on top of each one.
Add a dollop of rhubarb marmalade, and top each piece with a tiny mint leaf.