Whisk flour, baking powder, salt, and cardamom together in a medium bowl.
In the bowl of an electric mixer, beat butter and sugar together until creamy and pale in color.
Add eggs one at a time, incorporating one fully before adding the next.
Beat in rosewater to fully incorporate.
With mixer on low speed, add ⅓ of flour mixture, mixing until just incorporated (do not over-beat).
Follow with half of buttermilk, then second third of flour, the other half of buttermilk, then the remaining flour, mixing completely between each addition.
Spoon batter over arranged dates, spreading evenly and taking care not to jostle dates.
Bake cake in middle rack of oven 30-40 minutes, or until a tester comes out clean.
Cool cake in pan on a rack for about five minutes.
Run a knife around edges of cake, then invert onto serving plate, replacing any dates that stick to pan.
Garnish with edible flowers, ground pistachios, or dried rose petals, and serve warm or at room temperature.