1 C. granulated sugar
2 C. sliced rhubarb (1/2 in. thick)
1 tsp. grated lemon peel
2 T. lemon juice
1/2 C. plus 2 T. butter
1/3 C. firmly packed brown sugar
2 T. bourbon or orange juice
1/2 C. pecan halves
2 large eggs
1 1/2 tsp. baking powder
1 tsp. vanilla
1/4 tsp. salt
1 carton (7 1/2 to 8 oz.) crème fraîche or sour cream
2 C. all-purpose flour

In a bowl, combine 1/4 C. granulated sugar, the rhubarb, lemon peel, and 1 T. lemon juice. In a 10-inch frying pan over medium heat, melt 2 T. butter; add brown sugar, bourbon, and remaining 1 T. lemon juice; stir often until caramel has thickened and is medium brown, 7 to 10 minutes. Spread in a buttered 9-inch round cake pan (with 2-in.-tall sides). Arrange pecans over caramel. Scatter rhubarb mixture on top. In a bowl, with a mixer on medium speed, beat remaining 1/2 C. butter and 3/4 C. granulated sugar until blended. Beat in eggs until smooth. Beat in baking powder, vanilla, and salt, half the crème fraîche, then half the flour; repeat with remaining crème fraîche and flour. Drop spoonfuls of batter over rhubarb; spread smooth. Bake in a 350° oven until cake is golden on top and a toothpick inserted in center comes out clean, 40 to 50 minutes. Let cool on a rack 15 minutes. Run a knife between cake and pan sides. Invert onto a plate. Serve warm.