Slice and arrange the rhubarb in a 9'' springform pan. Sprinkle 1/2 cup sugar over the top, and then pour on melted butter. Set aside.

Cream the butter and sugar together until light and fluffy, about 2-3 minutes. Add the eggs, one at a time, beating after each addition.

Add in vanilla and yogurt.

In a separate bowl, stir together baking powder, spices and flour. Add into the wet mixture and stir until just combined. The batter will be thick.

Spoon the batter on top of the rhubarb mixture in a springform pan and smooth out the top with a spatula.

Bake at 350 for 40 to 45 minutes. If you're using an 8'' cake pan, the cake will be deeper and may take up to an hour to cook through. Test it in the center with a toothpick, but make sure you don't poke down into the rhubarb layer, which will remain gooey even when cooked.

Once cooked, remove the cake from the oven and allow to cool for 5 minutes. Remove the springform outside of the pan and invert the cake onto a plate.  

Allow the cake to cool completely before serving.