1. Add the butter to a microwave-safe bowl and melt in the microwave – about 1 minute on high. Add the brown sugar and heavy cream and mix to combine. Continue heating up in the microwave in two to three 20-second intervals, mixing in between until you get a thick sauce. Set aside to cool down a bit.

2. Preheat the oven to 350°F (180°C). Grease an 8 inches (20cm) baking tin and line with a piece of baking parchment. Pour the sauce on top, spread evenly, and sprinkle the rhubarb and berries evenly on top. Set aside.

3. Add the butter, sugar (or xylitol), and vanilla extract to a large bowl and mix until light and fluffy. Add the eggs, one after another, and mix well after each addition. Mix the flour with baking powder and salt, add together with the milk to the bowl and until just combined. Add the jam and fold in. Pour the batter on top of the rhubarb and berries, smooth out the top and bake for about 35-45 minutes or until a toothpick inserted into the center of the cake comes out clean. Take out of the oven and let cool down for about 10 minutes, then loosen the cake from the baking tin and invert it onto a serving plate. Serve warm or cooled down with some vanilla ice cream or whipped cream.