Make the Streusel Topping:

1. Preheat oven to 350F (Conventional) or 330F (Convection, fan). 
2. In a small bowl, combine the cubes of butter, sugar and flour. Using your fingers, rub the dry ingredients into the butter until they butter pieces are various sizes and flattened a bit. Alternatively, cut in butter with a fork or pastry cutter. Stir in the almonds. Set aside.

Mix the Cake Ingredients:

1. First, combine the dry ingredients in a large bowl. Whisk together the flour, baking powder, baking soda and salt. 
2. Make the Brown Butter: Into the cast iron pan, place the butter over Medium to Medium High Heat. As it melts, it will begin to spatter, then will bubble and foam. Once the foaming begins to slow, keep an eye on it to prevent it from burning. The milk solids will begin to separate, and you need to remove the butter from the heat before those bits turn black. So watch for them to brown. This is when you should pour the brown butter into a bowl. Since the cast iron is dark, it’s difficult to see the bits, so I remove it from the heat when the remaining foam starts to brown. 
3. Combine the wet ingredients. Pour the butter into a medium bowl. Add the sugar and whisk or mix until combined. Add the egg, stir again until combined. Then add the buttermilk and vanilla and stir until combined.
4. Pour the wet ingredients into the dry ingredients. Stir rather gently until only tiny bits of flour are still visible. Then add the berries, stirring until they are nicely distributed.
5. Pour batter into the cast iron skillet. Sprinkle reserved topping over the top of the batter. Place in preheated oven.
6. Bake until topping is golden brown and a tester inserted into the center comes out clean or with only berry juice on it, 40-50 minutes. In my oven this took 43 minutes.