1. In a bowl, with a mixer on medium speed, beat butter with 3/4 cup sugar until smooth; beat in egg yolks. Stir in flour, cornmeal, baking powder, and salt until well blended. Divide dough in half. Press one portion over bottom and up sides to rim of a 9-inch fluted tart pan with removable rim. Place other portion on a lightly floured piece of waxed paper. With a lightly floured rolling pin, roll out dough to a 10-inch round. Slide round onto a baking sheet. Chill tart shell and round until firm, at least 30 minutes, or wrap airtight and chill up to 3 days.
2. In another bowl, mix cherries, tapioca, lemon juice, and remaining 1/4 cup sugar. Let stand 10 minutes. Pour into chilled tart shell and spread level. Invert round over tart. Press edges into rim of tart pan, pinching off any excess. Sprinkle tart lightly with sugar.
3. Place tart pan on an uninsulated baking sheet. Bake on the lower rack of a 375° oven until top is golden brown, 35 to 40 minutes, rotating tart halfway through baking time. Serve warm or at room temperature.