Preheat the oven to 375°F / 190°C and grease the base and sides of 24 cm tart pan with removable bottom with butter.
To make the crumble topping: whisk together the flour, sugar, salt, and cinnamon together in a mixing bowl until combined. Using a pastry cutter (or your hands), cut the butter into the dry mixture until it resembles coarse crumbs. Store in the fridge while you make the base of the cake.
To make the base: in a large mixing bowl whisk together the ingredients (flour, baking powder, baking soda, salt, and lemon zest). In a second mixing bowl, whisk together the buttermilk, lemon juice, maple syrup, oil and vanilla. Add these wet ingredients to the dry ingredients and stir with a spatula until JUST combined.
Place your plum halves cut-side-up arranged on top of the batter, then sprinkle the crumble mix over top. Transfer to the oven and bake for 40-45 minutes, or until the cake is golden and the crumble topping has browned.
NOTE: place a sheet of parchment paper or a baking tray under the tart form as the juices from the plums may bubble over as it bakes.
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