To minimize waste, reserve the egg white left from making the tart pastry for use in the filling. If your blackberries are either very tart or very sweet, adjust the amount of sugar in the filling by about a tablespoon or so . Wondra flour is sold in small canisters in the baking aisle alongside all-purpose flour. All-purpose can be used in place of Wondra, it will give the filling a slightly starchy, gritty feel. The tart is best eaten the day it is made. Leftovers, however, should be refrigerated and brought to room temperature before serving (unfortunately the crust will be soggy, but the flavor of the tart will be almost as good as when eaten fresh).
FOR THE TART PASTRY:.
Whisk together yolk, cream and vanilla in a small bowl, Combine flour, sugar and salt in food processor with four 1-second pulses. Scatter butter pieces over flour mixture; pulse to cut butter into flour mixture until it resembles coarse meal, about twenty 1-second pulses. With machine running add egg mixture and process until dough comes together, about 12 seconds. Turn dough onto sheet of plastic wrap and press into 6-inch disk; wrap with plastic wrap and refrigerate at least 1 hour or up to 48 hours.
Remove dough from refrigerator (if refrigerated longer than 1 hour, let stand at room temperature until malleable.) Unwrap and roll out between lightly floured sheets of parchment paper or plastic wrap to 11-inch round (if dough becomes soft and sticky, slip onto baking sheet and refrigerate until workable.). Transfer dough to tart pan by rolling dough loosely over rolling pin and unrolling over 9-inch tart pan with removable bottom. Working around circumference of pan, ease dough into pan by gently lifting dough with one hand while pressing dough into corners and sides of pan with other hand. Press dough into flute sides of pan, patching breaks or cracks if necessary. (If some edges are too thin, reinforce sides by folding excess dough back on itself). Run rolling pin over top of tart pan to remove excess dough. Set dough-lined tart pan on baking sheet or large plate and freeze 30 minutes. (Frozen dough-lined tart pan can be wrapped tightly in plastic wrap and frozen up to 1 month.).
Meanwhile adjust oven rack to middle position and heat oven to 375°F Set dough-lined tart pan on baking sheet; lightly spray one side of 18-inch square heavy-duty extra-wide foil with nonstick cooking spray. Press foil greased side down inside frozen tart shell, folding excess foil over edge of pan; fill with metal or ceramic pie weights. Bake until pastry appears dry and pale gold under foil and edges have just begun to color, 20-25 minutes, rotating halfway through baking. Remove from oven and carefully remove foil and weights by gathering edges of foil and pulling up and out Return baking sheet with tart shell to oven and bake until sides are medium brown, about 5 minutes; set on wire rack to cool.
FOR THE FILLING.
While tart shell is cooling, heat butter in small saucepan with light-colored interior over medium heat; Cook, swirling or stirring occasionally, until butter smells nutty and milk solids at bottom are golden brown, about 7 minutes. Transfer butter to small heatproof bowl to stop cooking; cool butter until just warm to touch. Whisk egg and egg whites in medium bowl until combined; Add sugar and salt, whisk vigorously until light colored, about 1 minute. Whisk in warm browned butter until combined; then whisk in vanilla, Kirsch, and lemon zest and juice. Whisk in Wondra flour, then whisk in cream until combined.
ADD BERRIES AND BAKE:.
Distribute blackberries in single, tightly packed layer in bottom of cooled tart shell, Pour filling mixture evenly over blackberries. Place tart on baking sheet in oven . Bake until fragrant and filling is set (does not jiggle when shaken), bubbling lightly around edges, and surface is puffed and deep golden brown ,about 30 minutes, rotating sheet pan after about 20 minutes for even browning. Cool on wire rack to room temperature, at least 1 1/2 hours or up to 6 hours. Remove tart pan ring; slide thin-bladed spatula between tart pan bottom and crust to loosen then slide tart onto serving platter.