Preheat oven to 375°F (190°C). Grease and flour a 9x9 inch baking pan or line with parchment paper, leaving an overhang for easy removal.

Prepare the Streusel: In a medium bowl, combine the flour, sugar, and cinnamon for the topping. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Set aside.

Prepare the Cake Batter: In a small bowl, whisk together the 1 3/4 cups flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, cream the softened butter and 1 cup of sugar until light and fluffy (about 3 minutes). Beat in the eggs one at a time, beating well after each addition. Mix in the vanilla extract.

Alternately add the dry ingredients and the sour cream to the wet ingredients, beginning and ending with the dry mixture (three additions of dry, two of wet). Mix only until just combined; do not overmix.

Gently fold in the chopped strawberries and rhubarb until evenly distributed throughout the batter.

Spread the batter evenly into the prepared pan. Sprinkle the entire surface evenly with the reserved streusel topping.

Bake for 45 to 55 minutes, or until a toothpick inserted into the center of the cake comes out clean. If the topping begins to brown too quickly, loosely tent the cake with foil.

Let the cake cool in the pan on a wire rack for at least 20 minutes before lifting it out using the parchment overhang. Serve warm or at room temperature.