Rhubarb Upside Down Cake

Ingredients :
For the Topping
* 4 tablespoon(s) unsalted butter, melted
* 1/2 cup(s) all-purpose flour
* 1/4 cup(s) sugar
* Coarse salt
For the Cake
* 1 1/2 stick(s) unsalted butter, room temperature, plus more for buttering pan
* 1 pound(s) rhubarb, trimmed and cut on a very sharp diagonal about 1/2 inch thick
* 1 3/4 cup(s) sugar
* 1 1/2 cup(s) all-purpose flour
* 1 1/2 teaspoon(s) baking powder
* Coarse salt
* 1/2 teaspoon(s) finely grated orange zest
* 1 tablespoon(s) fresh orange juice
* 2 large eggs
* 1 cup(s) sour cream

Directions :
1. Preheat oven to 350 degrees F. Make the topping: Stir together butter, flour, sugar, and 1/4 teaspoon 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 for 2 minutes. Toss again, and spread in pan.
3. Whisk together flour, baking powder, and 1 1/2 teaspoons salt. Beat remaining stick butter and cup 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 mixture 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.