Grease a 9 inch cake pan.

Trim the rhubarb to remove the leaves and brown bottoms. Slice the stalks horizontally so that you have thin stalks then slice to varying lengths until you have enough to fit across the bottom of the pan (some will be shorter; the ones that go in the center will be longer).

In a skillet, add the butter and set over low heat. Once it melts, add the brown sugar and stir until it comes together in a thick, shiny mix.

Spread the butter mix into the bottom of the prepared pan. Arrange and lay the rhubarb over it so that the redder sides of the stalks are face down, adding enough to cover the entire bottom of the pan. Set aside while you make the cake batter.

Preheat the oven to 350 F.

Set the butter in a heatproof bowl and microwave it until it’s mostly melted (but don’t let it sputter or you’ll lose some of the fat needed in the cake).

Add the vanilla, cinnamon, and salt to the butter and whisk. Add the sugar and whisk very well, for at least a full minute.

One by one, add the eggs - whisking well after each addition. The mix should be thick and smooth. Add the remaining ingredients (flour, baking powder and sour cream) and whisk to combine.

Pour the batter over the rhubarb in the pan and smooth out into an even layer.

Bake the cake for about 40 minutes, until a cake tester inserted into the center (not the rhubarb) comes out clean from batter.

Let the cake cool briefly (about 10 minutes) in the pan then loosen it with an offset spatula run around the sides. Set a plate over the pan and flip the cake then pat the bottom to make sure you get out all the rhubarb. Lift the pan - it’s ok if some of the sugar/butter mixture got stuck on the edges.

Serve the cake warm. Store leftovers in the fridge in an airtight container.