Preheat oven to 350°F (175°C). Grease and line an 8×8-inch pan with parchment, leaving a little overhang for easy lifting.

In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, nutmeg, ginger, and cloves.

In a separate bowl, whisk the granulated sugar, brown sugar, and oil until sandy and cohesive. Add eggs, pumpkin puree, sour cream, and vanilla. Whisk until smooth.

Fold the wet ingredients into the dry ingredients with a spatula until just combined. A few small lumps are fine; do not overmix.

In a bowl, stir together the flour, brown sugar, granulated sugar, cinnamon, and salt. Add melted butter and mix with a fork until clumpy.

Fold in nuts if using, creating big, chunky clusters.

Pour the batter into the prepared pan and smooth the top. Sprinkle the streusel evenly, squeezing some into larger clumps.

Bake for 35–42 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. The edges should look set and the streusel golden.

Cool in the pan for 20–30 minutes before preparing the glaze.

Whisk the glaze ingredients to a drizzle-able consistency and zigzag it over the top. Slice and serve warm or at room temperature.