Rhubarb Upside-down Cake

This cake was much more fun than the first rhubarb dessert I made.  It looked beautiful and tasted really good too.  Although I didn't make the most artistic pattern with the pieces of rhubarb, the shape of the pan I used made the cake appear prettier than it probably was.

1/2 c. brown sugar
4 Tbs. butter
5-6 stalks of rhubarb

5 Tbs. butter, melted
2/3 c. sugar
2 eggs
1/2 c. buttermilk
1 tsp. vanilla
1 tsp. baking powder
1/4 tsp. baking soda
1 1/4 c. flour
1/4 tsp. salt

Melt the 4 Tbs. of butter and let the brown sugar dissolve in it.  Pour this mixture into the bottom of your cake pan (I used a silicone pan, but aim for something about 9").  Arrange your sliced rhubarb on top of the butter/sugar mixture.  In a bowl, mix together the rest of the ingredients and pour over the rhubarb.  Bake at 350F for 30 minutes.  After it cools for a minute or two, flip the cake out onto a plate.