Preheat your oven to 350 degrees. Grease your 9-inch pan with butter or cooking spray. Line the bottom with parchment paper. This helps the cake come out easily later.

Mix the sugars, cinnamon, and nutmeg in a medium bowl. Pour in the melted butter and stir. Add the flour and mix with a fork. It should look like big, moist crumbs.

Place the soft butter and sugar in a large bowl. Beat them together for about three minutes. The mixture should look very pale and fluffy. This adds air to the cake.

Add the eggs one at a time while mixing. Then, stir in the vanilla, lemon zest, and sour cream. Mix until the batter looks smooth. It might look slightly curdled, but that is okay.

In a separate bowl, whisk the flour, baking powder, baking soda, and salt. Slowly add this to the wet ingredients. Mix on low speed until just combined. Do not overmix the batter.

Toss your blueberries with a tiny bit of flour first. Fold them gently into the thick batter using a spatula. Be careful not to crush the berries. This keeps the batter white.

Pour the batter into your prepared baking pan. Use a spatula to smooth the top. Make sure it reaches all the corners evenly. The batter will be quite thick and heavy.

Sprinkle the crumb mixture over the batter. Use your hands to cover every inch. Bake for 40 to 50 minutes. A toothpick in the center should come out clean.

Let the cake cool in the pan for 15 minutes. Lift it out and place it on a wire rack. Dust with powdered sugar if you like. Slice into squares and serve warm.