Preheat oven to 350°F (175°C). Grease an 8x8 inch baking pan.
Make the cake base: In a mixing bowl, whisk together flour, sugar, baking powder, salt, and cinnamon. Spread evenly in the prepared pan.
Prepare pumpkin filling: In a separate bowl, combine pumpkin purée, sugar, egg, cinnamon, ginger, nutmeg, cloves, and salt. Mix until smooth. Spread evenly over the cake base.
Prepare crumble topping: In a small bowl, combine flour, sugar, and cinnamon. Cut in cold butter using a fork or fingers until mixture resembles coarse crumbs. Add nuts if desired. Sprinkle evenly over the pumpkin layer.
Bake: Place in preheated oven for 35–40 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Allow cake to cool for 10 minutes before slicing. Serve warm or at room temperature.