Heat the oven to 350°F (175°C). Grease the pan and line with a parchment sling. Keep streusel ingredients separate from cake ingredients.
Combine streusel flour, brown sugar, and cinnamon. Cut in cold streusel butter until uneven crumbs form, then refrigerate while mixing the cake.
Whisk cake flour, baking powder, baking soda, and salt. Beat cake butter and sugar for 3 minutes until light and creamy.
Beat in eggs individually and add vanilla. Mix in half the dry ingredients, then sour cream, then remaining dry ingredients, using low speed.
Spread half the thick batter in the pan. Scatter one-third of the streusel over it, then spoon and gently spread the remaining batter.
Scatter remaining streusel across the top without pressing it down. Bake 35–45 minutes until a tester through the cake portion has no wet batter.
Let cool in the pan on a rack for about 45 minutes. The crumb and streusel become easier to cut as the cake settles.
Lift out and cut into nine squares with a sharp knife. Serve slightly warm or at room temperature, keeping the remaining cake covered after cooling.
