Preheat oven to 375°F. Heavily butter a 10-inch removable-bottom tart pan. Be sure to also butter between the two pieces, where the bottom lip touches the insert, because this is where the crust most often sticks when the rhubarb juices seep through the pastry and onto the tart pan.

Combine flour, sugar, baking powder, salt and butter in a food processor. Pulse to cut the butter into dry ingredients; the mixture will be the consistency of coarse sand. Add egg and egg yolks, and pulse briefly, until egg is evenly distributed and pastry clumps together. Gather pastry together with your hands and press it evenly onto the bottom—but not the sides—of the prepared tart pan.

In a medium bowl, combine diced rhubarb, sugar, cardamom, rose petals and rose water. Let sit for 10–15 minutes, until juices begin to form and mix well. Spoon rhubarb with juices onto the center of the pastry, and then distribute evenly to within ¾ inch of the edges. The edges must remain clear so that they can rise.

Place the tart pan on an edged baking sheet in the top third of the oven. Bake for 50–55 minutes, until pastry is dark golden brown at the edges and golden brown at the center.

Remove from oven and let cool for 10 minutes only, removing the outer edge of the tart pan while it’s still warm keeps it from sticking. Let tart cool or serve while still warm.