1) Prep the pan and oven
Heat oven to 350°F. Grease a 9x9-inch baking pan. For easy lifting (especially nice if you do the cream cheese swirl), line with parchment leaving a little overhang on two sides.
2) Make the streusel topping
In a bowl, whisk flour, brown sugar, cinnamon, and salt. Add cold cubed butter and cut it in using your fingers or a pastry cutter until you have a mix of crumbs and bigger clumps. Stop before the butter warms up too much. Pop it in the fridge while you make the batter.

3) Optional: mix the cream cheese swirl
In a small bowl, mix cream cheese and sugar until smooth. Add egg yolk and vanilla and mix again. Set aside.
4) Mix the dry ingredients
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
5) Cream butter and sugar
In a large bowl, beat softened butter and granulated sugar until light and fluffy, about 2 minutes. Beat in eggs one at a time, then mix in vanilla.
6) Finish the batter
In a small bowl or measuring cup, stir the sour cream and milk together until smooth. Add the dry ingredients in two additions, alternating with the sour cream mixture (begin and end with dry). Mix gently just until combined. Do not overmix. The batter will be thick, like a soft frosting.
7) Assemble
Spoon half the batter into the pan and spread evenly.
If using the cream cheese swirl, dollop it over the batter and lightly swirl with a knife.
Dollop the remaining batter on top in spoonfuls, then spread gently so you do not drag the swirl through the whole cake.
Sprinkle the streusel evenly over the top. For extra-craggy topping, squeeze a handful into bigger clumps and scatter them around.
8) Bake
Bake for 38 to 45 minutes, until the top is golden and a toothpick comes out with moist crumbs (cream cheese swirl may smear a bit if you hit it). If you use an instant-read thermometer, aim for about 200 to 205°F in the cake portion.
9) Cool, then glaze
Cool in the pan for 20 to 30 minutes so the crumb sets up, then whisk powdered sugar, 1 tablespoon milk, vanilla, and salt. Add more milk a teaspoon at a time until it drizzles easily.
Drizzle glaze over the warm-ish cake, slice, and serve.

Note: Ovens and pans vary, so start checking at 38 minutes if your pan is dark metal or your oven runs hot.