Step 1

Preheat oven to 350°. Butter a 9"-diameter cake pan. Line bottom with a parchment paper round; butter parchment. Combine buttermilk and saffron in a small bowl; let sit 5 minutes.

Step 2

Meanwhile, whisk flour, baking powder, salt, cardamom, and baking soda in a medium bowl to combine.

Step 3

Using an electric mixer on medium speed, beat granulated sugar and remaining 6 Tbsp. butter until light and fluffy, about 3 minutes. Add eggs one at a time, beating after each addition until combined and scraping down sides of bowl as needed. Reduce mixer speed to low and beat in half of dry ingredients. Add buttermilk mixture and beat just to combine, then mix in remaining dry ingredients (be careful not to overmix batter). Gently fold in 1 cup blueberries.

Step 4

Scrape batter into prepared pan and smooth surface. Sprinkle remaining 1 cup blueberries over top of cake, then sprinkle with sanding sugar. Bake cake until a tester inserted into the center comes out with a few moist crumbs attached, 50–65 minutes. Transfer pan to a wire rack and let cake cool 20 minutes. Turn out cake onto rack, then turn right side up.

Step 5

Serve slices of cake warm or at room temperature, topped with sweetened sour cream and more blueberries if desired.