In a medium bowl, whisk together brown sugar, flour, cinnamon, and salt. Stir in the melted butter and vanilla until you get big, clumpy crumbs. Set aside.
Grease and line an 8×8 or 9×9 square baking pan with parchment paper.
In a large bowl, cream softened butter and granulated sugar until light and fluffy (about 2–3 minutes). Add eggs one at a time, beating well after each. Stir in vanilla and sour cream.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt. Add to the wet ingredients and mix until just combined—don’t overmix.
Pour the batter into the prepared pan and smooth the top. Gently press the crumb topping evenly over the batter. Don’t be shy—it’s a thick layer!
Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean. If the top starts to brown too quickly, tent loosely with foil halfway through.
Let the cake cool for at least 20 minutes before slicing. Dust with powdered sugar if you want to get fancy.
