In a small bowl, combine blueberry jam, ground cardamom, and vanilla extract. Stir well and set aside.

In a large mixing bowl, whisk together flour, brown sugar, salt, baking powder, and ground cardamom.

In a separate bowl, mix together milk, sour cream, oil, eggs, and vanilla extract until fully combined.

Pour the wet ingredients into the dry ingredients and mix gently until combined; lumps are okay.

Preheat your oven to 400°F (200°C) and prepare the muffin tin. Fill each muffin cup halfway, spoon in blueberry jam, then swirl and top with remaining batter.

Let the muffin batter sit at room temperature for about 20 minutes for optional rise.

Bake for 15 to 18 minutes until tops are golden brown and a toothpick comes out clean.

Let cool for about 5 minutes, then transfer to a wire rack. Optionally prepare a glaze and drizzle over muffins.