Mother Day '10 Rhubarb Upside Down Cake

I recently subscribed to Martha Stewart's "Living" magazine. I know it's horribly cliche to be gay and a Martha lover but the woman knows her stuff! This recipe comes from her May 2010 issue (reprinted without permission).

Rhubarb Upside-Down Cake

Actual Time: 25 min
Total Time: 2 Hours 15 min
Makes: One 9-Inch Cake: Serves 10

This cake has a crumb "topping" that actually ends up on the bottom. Each bite has a surprisingly crunchy texture.

For the Topping

4 tablespoons unsalted butter, melted
1/2 cup All-purpose flour
1/4 cup sugar
1/4 teaspoon coarse salt

For the cake

1 1/2 sticks unsalted butter, room temperature, plus more fir buttering pan
1 pound rhubarb, trimmed and cut on a very sharp diagonal about 1/2 inch thick
1 3/4 cups sugar
1 1/2 cups all-purpose flour
1 1/2 teaspoons baking powder
1 1/2 teaspoons coarse salt
1/2 teaspoon finely grated orange zest, plus 1 tablespoon fresh orange juice
2 large eggs
1 cup sour cream

1. Preheat oven to 350 degrees. Make the topping: Stir together butter, flour, sugar, and salt until moist and crumbly.
2. Make the cake: Butter a 9-inch round cake pan (2 inches deep). Dot with 4 tablespoons butter (cut into pieces). Toss rhubarb with 3/4 cup sugar; let stand 2 minutes. Toss again, and spread in pan.
3. Whisk together flour, baking powder, and salt. Beat remaining stick butter and cup of sugar with a mixer on medium speed until pale and fluffy. Beat in zest and juice. Beat in eggs, 1 at a time, until incorporated, scraping down sides of bowl. Beat in flour mixtures in 3 additions, alternating with sour cream, until smooth. Spread evenly over rhubarb. Crumble topping evenly over batter.
4. Bake until a toothpick inserted into the center comes out clean and top springs back when touched, about 1 hour. Let cool for 10 minutes. Run a knife around edge of cake, and invert onto a wire rack. Let cool completely.

Note: Let cake cool for 10 minutes before removing it from the pan The rhubarb will be too hot to handle safely right after baking. But if the cake sits much longer, it may stick.