For the streusel: Adjust oven rack to middle position and heat oven to 350 degrees. Spray 8-inch square baking pan with vegetable oil spray. Combine brown sugar, flour, cinnamon, and salt in bowl. Stir in melted butter and mix until no dry spots remain and mixture forms clumps; set aside.
For the cake: Whisk flour, granulated sugar, baking powder, baking soda, and salt together in large bowl. Whisk milk, melted butter, egg and yolk, and vanilla together in separate bowl, then add to flour mixture and stir until just combined.
Pour batter into prepared pan and smooth top. Crumble streusel into pea-size crumbs evenly over top, pressing slightly to adhere. Bake until toothpick inserted in center comes out with few moist crumbs attached, 45 to 50 minutes. Let cake cool completely in pan, about 2 hours.
