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1
In a small pot, combine rhubarb, sugar and cornstarch. Stir to coat all the pieces with sugar and cornstarch. Add the water and place the pot over medium-high heat and bring the mixture to a bubble. Let cook, stirring occasionally, until the mixture has thickened and the rhubarb breaks down, about 5 minutes. Once thickened, pour into a bowl and chill in the fridge for at least 2 hours.
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2
Preheat oven to 350u00b0F and line the bottom of a 9-inch springform pan with parchment paper.
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3
In a small bowl, whisk together the 1 1/2 cups flour, baking soda and salt. Set aside.
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4
In a stand mixer, using a paddle attachment, cream the softened butter with the sugar. Once combined, add the sour cream. Once mixed, add the eggs one at a time. Little by little, add in the dry ingredients until well incorporated. Then, slowly stream in the buttermilk and vanilla extract. Scrape the batter into the springform pan and carefully spread into an even layer.
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5
In the same bowl you mixed the dry ingredients, make the crumble. Put remaining 6 tablespoons butter, remaining 1 cup flour, brown sugar, flaky sea salt, ginger and cardamom into the bowl. Using your hands, mix it all together until it forms big clumps. Set aside.
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6
Pour the cooled rhubarb sauce of top of the cake. Spread the mixture leaving a 1/2-inch border around the sides. Pile the crumble on top of the sauce layer. Do not pat it down.
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7
Place the cake into the oven and bake for 1 hour, or until the top is brown and the sides of started to pull away from the edges of the pan.
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8
Let the cake cool thoroughly before removing it from the pan.
