Preheat oven to 350°F (175°C). Grease and flour a 9-inch round or square pan.
Whisk flour, baking soda, baking powder, cinnamon, nutmeg, and salt in a bowl.
Beat softened butter with both sugars until light and fluffy (about 2–3 minutes).
Mix in egg and vanilla. Stir in pumpkin puree and sour cream until smooth.
Fold dry mixture into wet until just combined—don’t overmix.
Mix flour, sugars, cinnamon, and salt. Cut in cold butter until crumbly.
Spread batter in pan. Top evenly with streusel.
Bake for 35–40 minutes, until a toothpick comes out clean and top is golden.
Cool in pan for 10 minutes, then on a rack. Serve warm or at room temp.