1. Preheat oven to 400 degrees. Butter an 8-inch-round, 2-inch-deep cake pan and line bottom with parchment; butter parchment. Place 1 cup yogurt in a fine-mesh sieve set over a bowl; let drain in refrigerator.

2. Meanwhile, toss together rhubarb, 1/2 cup granulated sugar, 1 tablespoon butter, and 1/4 teaspoon salt on a rimmed baking sheet. Roast, stirring once, just until tender, about 15 minutes. Reduce oven temperature to 325 degrees.

3. In a food processor, finely grind pistachios. Add flour, baking powder, and remaining 1 1/2 teaspoons salt; pulse to combine. Beat together remaining 1 stick butter and 1 cup granulated sugar with a mixer on medium-high speed until light and fluffy. Beat in eggs, one at a time, then orange-blossom water. Reduce speed to low and beat in flour mixture in two batches, alternating with remaining 1 cup yogurt.

4. Transfer batter to pan and smooth top. Arrange about half of rhubarb over top; refrigerate remaining rhubarb in syrup until ready to serve. Sprinkle cake with granulated sugar and bake until a tester inserted in center comes out clean, about 1 hour, 15 minutes. (If browning too quickly, tent with foil.) Let cool in pan on a wire rack 15 minutes. Run a small sharp knife around edges. Invert onto a cutting board, then immediately flip onto rack; let cool completely.

5. Stir confectioners' sugar into drained yogurt. Chop remaining rhubarb and swirl into yogurt, along with some syrup. Serve cake with yogurt, drizzled with more syrup.