Preheat the oven to 350 degrees and grease a 9-inch cake pan. Prepare the caramel layer: In a medium saucepan, cook the sugar over medium heat, stirring frequently, for 3 to 4 minutes or until an amber caramel has formed. Add the softened butter and cook, stirring constantly until combined, 1 to 2 minutes. Immediately pour caramel mixture into the prepared cake pan, spreading to cover the entire bottom of the pan. Arrange the jackfruit so that it covers the caramel and the bottom of the pan.

Make the cake batter: In a small bowl, mix together the flour, baking powder and salt. In a medium bowl, whisk together the brown sugar, melted butter, vanilla, eggs, and yolks. Fold the dry ingredients into the wet and stir gently to form a smooth batter. Pour the batter over the jackfruit in an even layer in the cake pan, smoothing with a spatula as necessary.

Bake until the top of the cake is golden brown, approximately 35 to 40 minutes. Remove the cake from the oven and let it slightly cool on a wire rack for 10 minutes. Then gently invert the cake onto a plate.

Prepare the caramel drizzle: In a medium saucepan, heat the butter and brown sugar over medium heat, stirring frequently, until both have melted. Add water and vanilla, bring mixture to a rapid boil for one minute, then remove from heat and cool for 10 to 15 minutes. Drizzle over the inverted cake and serve warm or at room temperature.