• Preheat oven to 350° F.
• Grease or paper-line the wells of 18 muffin standard-size cups.

Topping

• Whisk oats, flour, brown sugar, cinnamon, and salt to combine.
• Toss in the butter to coat. With a pastry blender or clean fingers, work butter into the dry ingredients until the butter is evenly distributed and the mixture is crumbly.

Muffins

• In a large bowl, whisk together flour, baking soda, cinnamon, cardamom, and salt.
• In a separate bowl, whisk together eggs, sugar, sour cream (or yogurt), and vanilla. Add the melted butter and whisk well to combine.
• Add half of the dry ingredients to the wet, whisking until the mixture begins to come together. Add the remaining dry ingredients and stir the mixture with a silicone spatula until it just comes together. Remove the raspberries from the freezer and fold them in gently.
• Scoop the batter into the prepared pans with an ice cream scoop or large spoon.
• Divide the streusel topping over the muffin batter and sprinkle with coarse sugar, if desired.
• Bake until a toothpick inserted into the center of a muffin comes out clean, about 20 to 25 minutes.
• Let the muffins cool in the pans for 10 to 15 minutes before removing to a cooling rack. Serve warm or at room temperature.