Preheat oven to 350°F. Spray a 10-inch springform pan with cooking spray.
Whisk together flour, sugar, baking powder, and salt.
Using a pastry cutter, fork, or your fingers, work the butter into the flour mixture until it forms fine crumbs.
In a separate small bowl, whisk egg, buttermilk, pumpkin purée, and vanilla until combined.
Mix the wet ingredients into the dry until combined, being careful to not overwork the batter.
Pour the batter into the prepared pan, smoothing the top. Cover the top of the cake with the streusel.
Bake for 40-45 minutes. The top will be golden and a toothpick will come out clean.
Allow to cool on a wire rack in its pan before unmolding.