Preheat oven to 375° F. Grease a 9-inch square baking dish; set aside.
PREPARE THE TOPPING:
In a medium bowl, use a fork or your fingers to combine flour, granulated sugar, brown sugar, cinnamon, salt and nutmeg. Add melted butter and combine until crumbly. Refrigerate until ready to use.
PREPARE THE BATTER:
In a large bowl, cream together sugar and butter. Add the egg, vanilla extract and almond extract; mix well. Add the buttermilk; mix to combine.
In a separate bowl, sift together the flour, baking powder and salt. Add the dry ingredients to the sugar mixture; mix just until combined (do not over-mix). Fold in 1 cup of the blueberries.
Transfer batter to the prepared pan. Sprinkle remaining 1 cup of blueberries on top; press down gently. Add the crumb topping over all.
BAKE THE BUCKLE:
Bake the buckle in the 375° F oven for 35-45 minutes, or until a toothpick inserted in the center comes out clean.
Notes
Line the baking dish with an overhang of parchment paperand grease well if you'd like to be able to lift the entire cake out of the pan before slicing (as shown here). Otherwise, just grease the pan well and cut out slices directly from the dish.
Add half of the berries to the batter, and reserve the other half for the top. This way you have sweet, juicy berries in every bite, and you can see the beautiful fruit as the topping buckles.
If you're worried that the berries in your batter will sink, you can toss that first cup of blueberries with about 1-2 teaspoons of flour before stirring them in. I don't think this is necessary, though. The berries don't sink much in the thick batter!
Since the batter is very thick, I find that it's easiest to spread it in an even layer in the pan if you moisten your hands or spatula with a little bit of water first.
The total baking time will vary depending on your individual oven, as well as the type of baking dish that you're using. For instance, metal pans or a cast iron skillet will bake the cake faster than a glass or ceramic dish (shown here). As a result, it's best to check the cake after about 35 minutes, and pull it out when a toothpick inserted in the center comes out clean.
If you're offering the cake for dessert, serve a warm slice with a scoop of creamy vanilla ice cream for the ultimate treat!