Preheat your oven to 175°C (350°F), grease a 9-inch square baking pan, and line with parchment for easy removal.
In a large bowl, whisk flour, baking powder, baking soda, cinnamon, nutmeg, and salt until evenly blended.
In another bowl, beat butter, brown sugar, and granulated sugar until fluffy. Then, add eggs one at a time, followed by pumpkin puree, milk, and vanilla extract.
Combine wet and dry ingredients until just mixed. Avoid overmixing to maintain tenderness.
Prepare streusel by mixing flour, brown sugar, and cinnamon with melted butter until crumbly.
Spread half of the batter into the pan, sprinkle half of the streusel, add the remaining batter, and top with the last of the streusel.
Bake for 45–50 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool for at least 20 minutes before slicing.