Rhubarb Upside-Down Cake

I've got THREE serious posts building themselves in my notebook. But my head hurts and I'm not making any progress on them today. Instead I'll post my new favorite recipe for rhubarb. (Proper rhubarb pie recipe here.)

This is from the Martha Stewart Living magazine - I'll admit it, I bought it just because it said "Martha's favorite rhubarb recipes" on the cover. She made it on her tv show too.

I made it with salted butter, because that's what I had in the house, and just adjusted the quantities of added salt. If you use unsalted butter, add 1/4 teaspoon of salt to the topping and the cake. Also, I didn't have a 2 inch high pan, so I just used my tallest cake pan, and miraculously it didn't spill over onto the bottom of the oven. But if you have a taller pan, I recommend it.

Rhubarb Upside-Down Cake (serves 12 pieces)

Topping:
4 tablespoons salted butter
1/2 cup all purpose flour
1/4 cup sugar

Cake:
12 tablespoons butter, room temperature plus more for buttering the pan
1 pound rhubarb, sliced 1/2" thick, on a very sharp diagonal, into long thin strips
1 3/4 cups sugar
1 1/2 cup flour
1 1/2 teaspoons baking powder
1 1/4 teaspoon salt
1/2 teaspoon orange zest
1 tablespoon orange juice
2 large eggs
1 cup sour cream

1 Stir the topping together. (It may be runny, rather than crumbly at first, but when the butter cools off, you can crumble it.)
2. Butter a 9 inch round, 2 inch deep cake pan. Dot the bottom of the pan with 4 tablespoons of butter cut into tiny pieces.
3. Stir the rhubarb together with 3/4 cup of the sugar, let it sit for two minutes, then stir it one more time and lay the slices into the bottom. If a fancy arrangement matters to you, now is your chance. Remember this will be on the top when you're done.
4. Whisk together the flour, baking powder and salt.
5. Beat the remaining 8 tablespoons (one stick) of butter with the remaining 1 cup sugar until pale and fluffy. Beat in the zest and the orange juice, then the eggs, one at a time, scraping down the bowl between each addition.
6. Beat in the flour in three additions, alternating with the sour cream, until smooth.
7. Spread the batter over the rhubarb, then crumble the topping evenly over the batter.
8. Bake until a toothpick inserted in the center comes out clean and the top springs back when touched, about 1 hour.
9. Let the cake cool in the pan on a rack for ten minutes, then run a knife around the edge and invert it onto a wire rack. Cool completely. (If you take it out of the pan too soon, the rhubarb will be too runny, and too late, it may stick. Don't worry too much about this.)

We ate it for dessert while it was still just barely warm. Divine! On the show, Martha said somebody she knew served this cake with creme fraiche, and I'm sure that would be delightful. Or a little whipped cream, or vanilla ice cream, all good. But just by itself, it's delicious!