To prepare the filling, combine the blueberries, 2 tablespoons of sugar, lemon juice, and cornstarch in a 4-quart saucepan over medium heat on the stove. Stir, mashing more if desired, until the mixture begins to bubble and thicken. Once the mixture has thickened slightly, remove from heat to a separate container and place in the fridge to cool while you prepare the rest of the cake elements. This can be made ahead, covered, and stored in the fridge for up to one week.
To prepare the crumble topping, add the diced butter to a small saucepan or skillet set over medium heat. Stir with a whisk or swirl the pan occasionally to ensure the butter is melting evenly. Once melted, the butter will sizzle, foam, and eventually start forming little golden bits on the bottom of the pan. Continue cooking and stirring regularly until the butter has taken on an amber color and nutty aroma. Take care not to burn the butter. Remove the pan from heat and pour the brown butter into a medium sized mixing bowl.
Add the brown sugar, salt, and cinnamon to the butter. Fold in the flour until the mixture has crumbled. Set aside in the fridge while you prepare the cake. Alternatively, this can be made and stored in an air-tight container up to one week in advance.
To prepare the cake, preheat the oven to 350 degrees and prepare two (8-1/2” x 4-1/2” x 2-3/4”) loaf pans with cooking spray. You can also line the pan with two strips of parchment paper for easy removal, if desired.
In a large bowl or the bowl of a stand mixer, cream the room temperature butter on medium speed with a paddle attachment until the butter is lightened in color and smooth, about 2 minutes. Scrape the sides of the bowl and add the sugars. Cream for an additional 2 minutes.
Add each egg, one at a time, stirring well on medium speed after each addition. Scrape the sides of the bowl and add the vanilla, stirring to combine.
In a separate bowl, combine all the dry ingredients. Add half of the dry ingredients to the butter and egg mixture and stir on low just until combined. Add the buttermilk and stir on low to combine. Add the remaining dry ingredients and stir on low, folding with a spatula to finish combining.
Scoop one cup of batter into the bottom of each prepared pan and spread it out. Drizzle two tablespoons of the blueberry filling evenly over top of each battered pan. Repeat this process with the layering of batter and filling once more and then divide the remaining cake batter among the two pans. You will not use all the blueberry filling. Drag a butter knife through the pans, back and forth several times, to swirl in the blueberry filling. Top each pan with 1/2 of the crumble.
Bake in a preheated oven for about 45-55 minutes, or until a toothpick inserted comes out clean. Allow to cool briefly before eating, or cool completely to remove the entire cake from the pan.