Like miniature shortcakes, these tarts are easy to prepare in advance. If you intend to serve them outdoors in warm weather, serve the whipped cream on the side.
For the dough, beat the butter and sugar with the paddle on medium speed until very soft and light. Beat in the vanilla and the yolk and continue beating until smooth and shiny, about 3 more minutes. Stop the mixer, sift the cake flour and add to the butter mixture. Pulse the mixer on and off to incorporate the flour. Scrape the dough onto a piece of plastic, wrap and chill it until firm. Roll the chilled cookie dough 1/8-inch thick on a floured surface. Line 4-inch tart pans with the disks of dough. Fill the tart shell with dried beans and bake at 325 degrees about 25 to 20 minutes, until pale golden. Cool off the pans. Remove from pans and remove beans. Arrange on a clean, paper-covered pan. Whip the cream with the sugar until it holds its shape. Spread jam in the bottom of the tart shells. Make alternating concentric rows of raspberries, blueberries and piped rosettes of whipped cream in the tart shells. Pipe the whipped cream with a pastry bag fitted with a 1/2-inch star tube (Ateco #4). Serve the rest of the cream on the side.