170 g butter, softened, (1 1/2 sticks)
454 g rhubarb, cut on a sharp diagonal 1.5 cm/1/2 inch thick
250 g sugar
135 g all-purpose flour
50 g whole wheat pastry flour
7 g baking powder, (1 1/2 tsp.)
⅛ tsp. salt
0.5 tsp. orange zest
1 tbsp. Grand Marnier
235 g sour cream, (1 cup)
57 g butter, melted, (4 tbsp.)
60 g all-purpose flour (or whole wheat pastry flour)
50 g sugar
1 pinch salt
Preheat oven to 350 F. Butter 9-inch round cake pan. Dot with 57 g/4 tbsp. butter flakes.
For the topping: Stir together butter, flour, sugar and salt, until moist and crumbly.
Toss rhubarb with 160 g sugar; let stand for 2 min. Toss again and spread in pan.
In a bowl, sift together flour, baking powder and salt.
With a mixer, beat 113 g/1 stick butter and remaining 90 g sugar on medium speed until pale and fluffy. Beat in orange zest and Grand Marnier. Beat in eggs, one 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.
Bake for ca. 1 hour, or until a needle comes out clean and top springs back when touched. Let cake cool for 10 min. Run knife or plastic spatula around edge of cake and invert onto a wire rack.