Heat oven to 350°F. Thoroughly grease your cake tin, most especially thoroughly if using a bundt pan.
In a mixing bowl whisk together the flour, baking powder, baking soda, salt, herb de provence, and sugar.
In a separate bowl whisk together the grapefruit juice, olive oil, buttermilk, rose water, and eggs.
Pour the wet ingredients into the dry and gently stir with a spoon to just combine, making sure to scrape the bottom. Do not overmix.
Pour the batter about 2/3 way up the sides of your cake tins or pan (will make about 3 small bundt cakes or 2-3 8″ layers depending on how thick you make them), and bake at 350°F for 30 minutes to an hour (depending on size of cakes). Cake should be a deep golden brown and a cake tester should come out clean when inserted. A few crumbs are fine; it just shouldn’t be wet or goopy.
Allow cakes to cool in their tins about 10 minutes, and then turn out onto a cooking rack to cool completely before icing.
Meanwhile whisk together your glaze. In a bowl whisk the juice and rosewater into the powdered sugar. Add sugar if you need to thicken it to get it to a pourable but quite thick consistency. Alternately, you can thin it with additional juice or cream as needed. Spoon the glaze over your cake. And enjoy!