Place 12 muffin liners in a muffin pan. Use nonstick spray to spray the liners. Preheat the oven to 375°F or 190°C.
In a medium bowl, combine all the dry ingredients and whisk with a fork. Set aside.
In a small bowl, put the tablespoon of light brown sugar for your topping as well as walnut crumbs from chopping if you have them! Set aside.
In the bowl of a stand mixer or a large bowl with a hand mixer, beat the butter and sugars together until creamy. Add the eggs, vanilla extract, and sour cream. Mix until all are combined, scraping down the sides as necessary.
Add half the dry ingredients to the egg mixture, mixing until almost combined. Add the milk and briefly mix (less than 10 seconds), then add the rest of the dry ingredients. Mix until just combined.
Using a spatula, fold in the raspberries and walnuts. The batter should be very fluffy. Scoop the batter evenly into the muffin liners. The batter should come almost to the top of each liner. Sprinkle the brown sugar topping evenly on the tops of the muffins.
Bake for 18-20 minutes. Let cool for 1-2 minutes before using an offset spatula or knife to pick up the muffins and move them to a cooling rack.