Preheat oven to 350°F (175°C) and grease a 9-inch round or square cake pan.
In a bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, ginger, nutmeg, and espresso powder.
Beat butter, pumpkin puree, and sugar until smooth, then add eggs and vanilla. Mix well.
Add the dry ingredients alternately with sour cream. Mix until just combined.
Transfer batter to the prepared pan.
For the streusel, combine oats, brown sugar, flour, butter, and cinnamon in a bowl.
Sprinkle streusel over the batter.
Bake for 35-40 minutes or until a toothpick comes out clean.
Let the cake cool on a rack before serving.