Thick, buttery crumb topping over a tender vanilla cake makes this a New York-style breakfast icon. Serve generous squares to the whole family with fresh coffee.
Preheat oven to 350°F. Grease a 9x13-inch baking pan liberally to ensure easy release.
In a mixing bowl, cream 4 oz unsalted butter and granulated sugar until light and fluffy, about 4 minutes, using medium speed.
Add eggs one at a time, mixing well between additions, then pour in milk and vanilla extract; mix until just combined. Scrape down bowl to avoid streaks.
Whisk together all-purpose flour and baking powder, then add to the wet ingredients. Mix gently until the batter is smooth but do not overmix.
Pour batter into the prepared pan, spreading evenly. In a separate bowl, combine the crumb topping ingredients: 5 oz melted butter, light brown sugar, flour, cinnamon, and salt. Use a fork to achieve large, moist crumb pieces; scatter evenly over the cake batter.
Bake for 38–45 minutes, checking for doneness at 40 minutes. The crumbs should be golden and the cake set. Avoid overbaking to maintain moisture.
Let cool in the pan for at least 15 minutes before slicing generous squares. Serve warm or at room temperature, preferably with fresh coffee.