In a saucepan over medium-low heat, melt your butter. Once melted, it will begin to foam — stir constantly with a rubber spatula. After a few minutes, it’ll start to smell nutty and you’ll see brown specks form at the bottom. That’s the toasted milk solids — pure gold. Remove from heat as soon as it turns amber-brown and smells nutty. Let it cool for 5–10 minutes before using.

In a large mixing bowl, whisk together your cooled brown butter, granulated sugar, and brown sugar until smooth and glossy. Add the eggs and vanilla extract and mix again until well combined.

Next, whisk in your pumpkin purée, milk, and sour cream until smooth and creamy.

In a separate bowl, whisk together flour, pumpkin pie spice, cinnamon, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until just combined — do not overmix.

Pour the batter into a greased or parchment-lined 9×9 baking dish.

In a small bowl, combine melted butter, brown sugar, flour, and pumpkin pie spice with a fork until crumbly. Sprinkle evenly over the batter.

Bake at 350°F (175°C) for 45–50 minutes, or until a toothpick inserted in the center comes out clean.

Let cool completely on a wire rack for about 1 hour.

In a small bowl, whisk together powdered sugar, maple syrup, cinnamon, and vanilla until smooth. Drizzle over cooled cake, slice into 16 squares, and serve.