For the Topping
6 tablespoons granulated sugar
1/4 cup dark-brown sugar
1 teaspoon cinnamon
For the Cupcakes
1 2/3 cups cake flour (not self-rising)
1/4 teaspoon baking soda
1 teaspoon baking powder
1/2 teaspoon salt
1 stick unsalted butter, softened
2/3 cup granulated sugar
2 large eggs
1 teaspoon pure vanilla extract
3/4 cup sour cream
6 ounces blueberries (1 1/4 cups)
For the Icing
1 1/4 sticks unsalted butter, softened
8 ounces cream cheese, room temperature
1/2 teaspoon pure vanilla extract
2 2/3 cups confectioners’ sugar, sifted
1/4 cup high-quality blueberry jam, such as Hero, strained (3 tablespoons)
Make the topping: Mix together granulated and brown sugars and cinnamon.
Make the cupcakes: Preheat oven to 375 degrees with rack in upper third of oven. Line cups of a standard muffin tin with baking cups. Sift together flour, baking soda, baking powder, and salt. Beat together butter and granulated sugar with a mixer on medium-high speed until pale and fluffy, 2 to 3 minutes. Beat in eggs, 1 at a time, then beat in vanilla, scraping down sides of bowl. Reduce speed to low, and beat in flour mixture in 3 additions, alternating with sour cream and beginning and ending with flour. Fold in blueberries.
Divide batter among baking cups. Add topping, pressing to adhere to batter. Bake until golden and a toothpick inserted in center comes out with moist crumbs attached, 20 to 21 minutes. Remove cupcakes from tin, and let cool completely on a wire rack.
Make the icing: Beat together butter and cream cheese with a mixer on medium-high speed until pale and fluffy, about 2 minutes. Reduce speed to low, and gradually beat in vanilla and confectioners’ sugar. Raise speed to medium-high, and beat 1 minute. Spoon jam on top of icing, and without stirring, spoon icing into a disposable pastry bag. Snip off a 3/4-inch opening in tip, and pipe swirled mounds of icing on top of each cupcake. Although best if eaten immediately, cupcakes can be refrigerated up to 6 hours and brought to room temperature 45 minutes before serving.