Preheat oven to 350°F. Grease a 9x13 inch baking pan.
In a medium bowl, whisk together the flour, baking soda and salt. Set aside.
In a large bowl, beat the butter and sugar together until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time, then stir in the vanilla and sour cream until combined.
Fold in the dry ingredients just until incorporated, being careful not to overmix. Gently fold in the diced rhubarb.
Spread the batter evenly into the prepared pan.
Make the streusel topping: In a small bowl, mix together the flour, brown sugar, butter and cinnamon with a fork until crumbly. Sprinkle evenly over the cake batter.
Bake for 40-45 minutes, until a toothpick inserted in the center comes out clean. Allow to cool completely before cutting into squares.
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