1. Preheat oven to 375°.
2. To prepare cake, combine rhubarb and 2 tablespoons flour in a medium bowl; toss well to coat.
3. Place brown sugar and 5 tablespoons butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add sour cream, rind, and vanilla; beat until well combined.
4. Lightly spoon 1 1/2 cups all-purpose flour and whole wheat flour into dry measuring cups; level with a knife. Combine flours, baking soda, 1 teaspoon cinnamon, and salt, stirring with a whisk. Gradually add flour mixture to butter mixture, beating at low speed just until combined. Fold in rhubarb mixture. This is a thick batter. Spread evenly into a 9-inch square baking pan coated with cooking spray.
5. To prepare streusel, combine turbinado sugar and 1/2 teaspoon cinnamon in a small bowl. Cut in 2 tablespoons butter with a pastry blender or 2 forks until mixture is crumbly; stir in nuts. Sprinkle streusel evenly over batter. Bake at 375° for 50 minutes or until a wooden pick inserted in the center comes out clean. Invert onto a cooling rack and cool completely. Cut into squares and enjoy.