Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a bowl, whisk together flour, baking powder, baking soda, cinnamon, ginger, and salt.
In another bowl, mix pumpkin, sugars, oil, eggs, vanilla, and milk.
Add dry ingredients to wet ingredients and stir until combined.
Pour the batter into the prepared pan.
In a small bowl, combine oats, brown sugar, cinnamon, and melted butter for the streusel.
Sprinkle the streusel over the cake batter.
Bake for 30-35 minutes or until a toothpick comes out clean.
Let the cake cool on a rack before slicing.