Blueberry Crumb Cake

Crumb Topping5 tbsp unsalted butter, at room temperature
1/4 cup sugar
1/3 cup (packed) light brown sugar
1/3 cup all-purpose flour
1/4 tsp salt
1/2 cup chopped walnuts

To make the crumb topping

1. Put all the ingredients except the nuts in a food processor and pulse just until the mixture forms clumps and curds and holds together when pressed.
   
2. Scrape the topping into a bowl, stir in the nuts and press a piece of plastic against the surface.
   
3. Refrigerate until needed. (Covered well, the crumbs mix can be refrigerated for up to 3 days.)
   

Cake
1 pint (2 cups) blueberries (preferably fresh, or frozen, not thawed)
2 cups plus 2 teaspoons flour
2 tsp baking powder
1/2 tsp baking soda
1/4 tsp salt
1/4 tsp ground cinnamon
1/8 tsp freshly grated nutmeg
2/3 cup sugar
grated zest of 1/2 lemon or 1/4 orange
3/4 stick (6 tablespoons) unsalted butter, at room temperature
2 large eggs
1 tsp pure vanilla extract
1/2 cup buttermilk

To make the cake

1. Center a rack in the oven and preheat the oven at 350 F.
   
2. Butter an 8" square pan (Pyrex is great for this) and put it on a baking sheet.
   
3. Using your fingertips, toss the blueberries and 2 tsp of the flour together in a small bowl just to coat the berries; set aside.
   
4. Whisk together 2 cups flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
   
5. Working in the bowl of a stand mixer or in another large bowl, rub the sugar and zest together with your fingertips until the sugar is moist and aromatic.
   
6. Add the butter and, with the paddle or whisk attachment, beat the sugar with the butter at medium speed until light, about 3 minutes.
   
7. Add the eggs one by one, beating for about 1 minute after each addition, then beat in the vanilla extract. Don’t be concerned if the batter looks curdled- it will soon smooth out.
   
8. Reduce the mixer speed to low and add the flour mixture and the buttermilk alternately, the flour in 3 parts and the buttermilk in 2 (begin and end with the dry ingredients.) You will have a thick, creamy batter.
   
9. With a rubber spatula, gently stir in the berries.
   
10. Scrape the batter into the buttered pan and smooth the top gently with the spatula. Pull the crumb mix from the refrigerator and, with your fingertips, break it into pieces. There’s no need ot try to get even pieces- these are crumbs they’re supposed to be lumpy and bumpy and every shape and size. Scatter the crumbs over the batter, pressing them down ever so slightly.
    
11. Bake for 55-65 minutes, or until the crumbs are golden and a thin knife inserted into the center of the cake comes out clean. Transfer the cake to a rack and cool just until it is warm or until it reaches room temperature.