For the caramel topping: Preheat the oven to 350 degrees F. Butter just the sides of a 9-inch round cake pan.
Stir the sugar and 1/3 cup water together in a medium saucepan over medium heat until the sugar dissolves. Cook, no longer stirring but gently swirling the saucepan occasionally, until the mixture is amber, 6 to 8 minutes. (If needed, brush any sugar crystals off the side of the pan with a pastry brush dampened with water.) Remove the saucepan from the heat, and carefully whisk in the butter (use caution; the mixture can bubble up considerably). Pour the caramel into the prepared cake pan, tilting the pan to cover most of the bottom (be careful; the pan will be very hot from the caramel).
Toss the rhubarb and strawberries in a medium bowl with the cornstarch, strawberry jam and a pinch of salt. Spoon on top of the caramel sauce and smooth into an even layer. Set aside.
For the cake: Whisk together the flour, baking powder, baking soda and salt in a medium bowl. Whisk together the milk and sour cream in a small bowl.
Beat the butter and sugar together with an electric mixer on medium-high speed in a large bowl until light and fluffy, about 4 minutes. Add the eggs 1 at a time, beating after each addition and scraping down the side of the bowl as needed, then beat in the vanilla. Reduce the mixer speed to low and add the flour mixture in 3 batches, alternating with the milk-sour cream mixture, and mixing until just incorporated.
Pour the batter over the rhubarb and strawberries in the cake pan and spread evenly. Bake until the cake is golden brown, bounces back when pressed and a toothpick or cake tester comes out clean when inserted in the center, 55 to 60 minutes. Run a knife around the edge of the cake to separate it from the pan. Let cool in the pan on a rack until cool enough to handle and almost room temperature, about 1 hour. Invert a serving platter over the cake pan and, while holding them together, quickly flip them over so the cake unmolds, fruit-side up. Let cool completely. Cut into wedges, and serve.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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