Make the Filling and Topping

In a food processor, pulse the walnuts, brown sugar, granulated sugar, and cinnamon until the walnuts are coarsely chopped. Remove and set aside 1¼ cups/6.3 ounces/178 grams to use for the filling.

To the remainder, add the flour, butter, and vanilla and pulse briefly to form a coarse, crumbly mixture for the topping. Scrape it into a medium bowl and refrigerate for about 20 minutes to firm up the butter to make it easier to crumble.

Preheat the Oven

Twenty minutes or longer before baking, set an oven rack in the lower third of the oven and preheat the oven to 350°F/175°C.

Mix the Liquid Ingredients

In a medium bowl, whisk the eggs, ¼ cup/2.1 ounces/60 grams of the sour cream, and the vanilla just until lightly combined.

Mix the Batter

In the bowl of a stand mixer fitted with the flat beater, mix the flour, sugar, baking powder, baking soda, and salt on low speed for 30 seconds. Add the butter and the remaining sour cream and mix on low speed until the dry ingredients are moistened. Raise the speed to medium and beat for 1½ minutes. Scrape down the sides of the bowl.

Starting on medium-low speed, gradually add the egg mixture in two parts, beating on medium speed for 30 seconds after each addition to incorporate the ingredients and strengthen the structure. Scrape down the sides of the bowl.

Using a silicone spatula, scrape about half of the batter (21 ounces/600 grams) into the prepared pan and smooth the surface evenly with a small offset spatula. Using your fingers, sprinkle lightly with the reserved 1¼ cups of crumb filling (do not press it into the batter). Drop the remaining batter in large blobs over the filling and carefully spread it evenly.

Bake the Cake

Bake for 40 minutes.

Meanwhile, use your fingertips to pinch together the crumb topping, breaking up the larger pieces so that about one-third of the mixture is formed into ¼ inch balls or clumps. (Do not make the balls larger because they will be hard to cut through when serving.) Let them fall onto a large piece of parchment and add the rest of the lightly pinched crumbs.

Remove the pan from the oven and gently place it on a wire rack. Scatter the blueberries onto the surface of the cake. Using the parchment as a funnel, quickly and evenly strew the crumb topping over the blueberries, which will sink slightly during baking. Continue baking for 15 to 25 minutes, or until a wooden toothpick inserted into the center comes out clean and the cake springs back when pressed lightly in the center. The crumb topping can make it difficult to test for doneness, so using an instant-read thermometer offers added assurance. The thermometer should read about 208°F/98°C.

Cool and Unmold the Cake

Let the cake cool in the pan on a wire rack for 30 minutes. Loosen the sides of the cake with a small metal spatula. Place a piece of plastic wrap that has been lightly coated with nonstick cooking spray on top of the cake and then a small folded dish towel. (The towel should come up to the rim of the pan to prevent the cake from splitting when inverting it.) Invert the cake onto a sheet pan. Reinvert it onto a cake board or baking sheet and cool completely, about 1½ hours.