Preheat oven to 350F. Grease a 9x13 inch pan and line with parchment, leaving an overhang for easy removal. For the streusel, combine flour, brown sugar, and cinnamon in a bowl. Work in the cold butter with your fingertips until clumps form, about 2 minutes. Set in the refrigerator.

Beat softened butter and granulated sugar on medium-high speed until light and fluffy, 3 to 4 minutes. Scrape down the bowl. Add eggs one at a time, beating well after each. Mix in sour cream, lemon zest, and lemon juice until smooth. The batter will look slightly curdled - this is normal.

Whisk together flour, baking powder, baking soda, and salt. Add to the wet ingredients on low speed and mix just until no dry streaks remain. Do not overmix. The batter will be thick.

Spread batter evenly in the prepared pan. It will be about 1 inch thick. Sprinkle the cold streusel evenly over the top, breaking up any large clumps. Bake until the top is golden and a toothpick inserted in the center comes out clean, 42 to 48 minutes.

Cool in the pan on a wire rack for at least 30 minutes before slicing. The cake cuts cleanly once fully cooled but is delicious slightly warm. Store covered at room temperature for up to 3 days.