Preheat oven to 350 degrees F. Line a 9x13inch baking pan with parchment paper, so that it hangs off on either side to make it easier to pull out later. Lightly grease with cooking spray and set aside.

To make the jam, cut the rhubarb into a 1-inch dice and place in a medium saucepan. Hull the strawberries and slice into a bowl. Using a potato masher or fork, roughly mash the berries until chunky. Throw into the pot with the rhubarb, along with the lemon juice, vanilla and salt. Cover and cook over moderate heat for 5 minutes. In a small bowl, whisk together the sugar and cornstarch and stir into the saucepan. Bring to a boil; cook and stir until thickened, about 2 minutes. Remove from the heat and set aside until ready to use.

To make the cake, in a large bowl, combine the flour, sugar, baking powder, baking soda, and salt. Add the butter and cut in with your fingers until the mixture resembles coarse crumbs with large bits of butter throughout. Beat buttermilk, eggs and vanilla in a separate bowl and then add to the dry ingredients. Stir until just combined.

Spread half of the batter into the prepared pan into an even layer. Carefully spread the jam over the top. Dollop the remaining batter over the top with a large spoon.

For the streusel, in a large bowl, combine the butter, flour and sugar with your hands until the mixture resembles coarse crumbs the size of peas. Sprinkle over the top of the cake and bake until golden brown and the center no longer jiggles, about 40 to 45 minutes. Remove from oven and let cool completely. Chill for about 30 minutes to make it easier to slice. Cut into squares and serve. Leftovers can be stored in the fridge, covered, for up to 4 days. Enjoy!