Preheat the oven to 325 degrees F.
Lightly grease a 4- by 14-inch rectangular tart pan with a removable bottom (I brush on melted butter), set it on a baking sheet and set aside. Alternatively, a 9 1/2-inch round tart pan with a removable bottom can be used.
Crush the rose petals with 2 tablespoons of the sugar using a mortar and pestle. Toss the raspberries with 1 tablespoon of the rose sugar in a small bowl, and then crush into a very coarse puree using the tines of a fork (pieces of fruit should still be discernible).
Combine the butter and remaining sugar in a large food processor and pulse until combined. Whisk together the eggs and vanilla, and then add to the processor and pulse to combine. Whisk together the almond flour, buckwheat flour, salt and baking powder in a medium bowl. Add to the food processor and pulse until the batter is thoroughly mixed.
Spread one-third of the batter into the prepared tart pan, and then gently spoon the crushed raspberries on top. Carefully spread the remaining batter over the raspberries. Scatter the remaining rose sugar on top (you may not use it all). Bake until the tart has set and turned a deep golden brown, 40 to 45 minutes. Let cool completely before unmolding and slicing. Store at room temperature, wrapped in plastic wrap, for up to 3 days.
For the whipped cream: Whip the cream using an electric mixer fitted with the whisk attachment. Add the sugar and continue to whip on high speed until it forms soft peaks. Gently fold in the creme fraiche and rose water using a flexible spatula. Keep chilled until ready to serve with the tart.
Almond flour isn't flour at all, it's actually just finely ground almonds. If you can't find any almond flour for purchase, you can make your own! Just grind a bag of whole, raw almonds in a food processor until they are a fine, flour-like consistency. If you can't find buckwheat flour, you can substitute with an equal amount of almond flour.