Combine flour, butter, sugar, salt, and lemon zest in a food processor and pulse until the mixture resembles coarse meal with some pea-sized lumps remaining.

Add egg yolks and process until just incorporated and the mixture begins to clump.

Turn the mixture out onto a work surface and divide into 4 portions.

Smear each portion once with the heel of your hand to distribute the fat.

Gather together 2 portions of dough and form into a ball; repeat with the other 2 portions.

Pat out each ball of dough with floured fingertips into a tart pan, creating an even 1/4-inch layer on the bottom and up the sides, extending about 1/8 inch above the rim.

Chill the tart shells for 30 minutes, or until firm.

Stir together sugar and cornstarch in a large bowl.

Add plums and lemon juice and toss to coat.

Let the plum mixture stand, stirring occasionally, for 30 minutes, or until juicy.

Preheat oven to 425F.

Arrange plum halves, skin sides down, in the tart shells, overlapping in a rosette pattern.

Halve any remaining plums lengthwise and tuck them randomly between the plum halves in the tarts.

Pour all juices from the bowl over the plums.

Bake the tarts in the middle of the oven for 15 minutes, then reduce the temperature to 375F.

Cover the tarts loosely with foil and bake until the plums are tender and the juices are bubbling and slightly thickened, 40 to 50 minutes more.

Brush warm juices in the tart over the plums.

Cool the tarts completely in the pans on a rack.