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 tart pan with removable rim. Place other portion on a lightly floured piece of waxed paper. Set aside.
In another bowl, mix cherries, tapioca, lemon juice and remaining 1/4 cup sugar. Let stand for 10 minutes as you roll out second portion of dough with a lightly floured rolling pin into a 10-inch round.
Pour cherries into tart shell. Invert round over cherries. Press edges into rim of tart pan, pinching off any excess. Sprinkle tart lightly with sugar.
Place tart pan on a baking sheet and bake on the lower rack of a 375 degree over until top is golden brown, about 35-40 minutes.