• Preheat oven to 375F. Line a 9-inch spring form pan with a circle of parchment paper at the bottom and butter the sides. Set aside.

to make the crumb topping

• In a small mixing bowl, combine the sugar, flour, butter, salt, cinnamon, and nutmeg. Use your hands to rub the ingredients together until you have a crumbly mixture. Set aside.

to make the cake

• In a small mixing bowl, combine the flour, baking powder, salt, and chopped walnuts. Whisk to combine, and set aside.
• In a medium mixing bowl, combine the sugar, egg, sour cream, buttermilk, and vanilla extract. Whisk well, then drizzle in the melted butter (make sure it's partially cooled and not too hot). Whisk together until smooth.
• Fold in the dry ingredients until mostly incorporated, but there are still a few dry streaks of flour left.
• Finally, add the frozen berries and gently fold the batter together until just combined, don't over mix.
• Spread the batter into your prepared pan, making it even. Top with the crumb topping.
• Bake for about 55-65 minutes until risen and golden.
• Remove from the oven and allow to partially cool in the pan before carefully removing.
• Serve warm or keep on the countertop for up to a day. Refrigerate for longer storage.