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1
Prepare the grapes: Separate the skins of the grapes from the center. Y squeezing them - reserve the center juicy parts in a medium saucepan and the skins in a medium bowl. Put the pan on the stove and , stir In the sugar - bring to a simmer over medium heat. Turn the heat down to low and cook, stirring occasionally, until the mixture breaks down, 4-5 minutes. Strain the juice to remove the seeds. Return the juice to the pot, this time, stirring in the reserved skins. Continue to cook over medium low heat until the mixture thickens to a jam consistency, 35 to 45 minutes. Set aside to cool to room temperature (the jam will thicken a bit more as it cools).
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2
Preheat the oven to 350u00b0F / 175u00b0C, with a rack in the middle. Use 43 g / 3 tablespoons of the butter to grease a 9-inch springform or a 9-inch round cake pan with sides at least 3 inches high. Line the bottom of the pan with a circle of parchment paper and grease the paper. Line a baking sheet with parchment.
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3
Spread the grape mixture in an even layer over the bottom of the pan and place the pan on the lined baking sheet.
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4
Make the cake: In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl, using a hand mixer), cream the butter and both sugars on medium-low speed until light and fluffy, 4 to 5 minutes.
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5
Add the eggs one at a time, mixing until each one is fully incorporated. Scrape the bowl well. Add the vanilla and mix to combine.
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6
In a medium bowl, whisk together the all-purpose flour, graham flour, baking powder, baking soda, cinnamon, and salt.
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7
Add one third of the flour mixture to the butter mixture and blend on low speed to combine, then add half of the creme fraiche, mixing to combine. Repeat, alternating the flour mixture and creme fraiche, until all of the ingredients are incorporated and the batter is smooth.
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8
Pour the batter into the prepared pan, spreading it evenly without disturbing the underlying bed of grapes. Bake the cake until a toothpick inserted in the center (but not deeply, or you'll poke into the juicy jam) comes out clean, 50 to 60 minutes. Cool the cake in the pan for 15 to 20 minutes.
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9
If you used a springform pan, remove the ring, invert the pan onto a wire rack and, while the cake is still warm, use the spatula to separate the base from the parchment and lift it away from the cake. Or, if you used a cake pan, run a small offset spatula around the outside of the cake and invert the cake onto the rack. Gently pull away the parchment; if any fruit is stuck to it, just pry it off and put it back where it belongs-it's easy to mash it back on there. Let cool completely.
