When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.) To cut the dough into perfect rectangles, make a template out of parchment paper, then use it to guide your chef's knife or pizza cutter. Toss the rhubarb and jam together just before assembling the tarts. If the mixture sits too long and starts to macerate, the filling will become too soft and juicy and make it difficult to assemble the tarts. Make sure to freeze the toaster tarts before baking. This will ensure that the filling cooks through and won't leak out of the pastry.