Preheat the oven to 350°F (175°C).
In a saucepan, melt butter, sugar and add the rhubarb, lemon juice, vanilla, and a pinch of salt. Cook until rhubarb is soft and caramelized.
Pour the rhubarb mixture into the bottom of a greased baking dish.
In a bowl, whisk together the eggs, olive oil, maple syrup, lemon zest, lemon juice, vanilla, cardamom, salt, and baking soda.
Gradually mix in the gluten-free flour blend until just combined.
Pour the batter over the rhubarb in the baking dish.
Bake for about 45-50 minutes or until a toothpick inserted in the center comes out clean.
Let the cake cool slightly before inverting it onto a serving platter.
To make the topping, melt ½ cup of butter and mix it with coconut oil, sugar, and vanilla extract. Pour over the warm cake.
Serve the cake warm with a dollop of whipped cream, if desired.
