3and 1/4 cups PLUS 2 tablespoons cake flour, divided
1and 1/2 teaspoons baking powder
2/3cupfresh blackberries (if you have any huge blackberries, cut them in half)
For the Triple Berry Frosting:
1/3cupfresh blueberries (frozen will also work; do not thaw first)
1/3cupfresh raspberries (frozen will also work; do not thaw first)
1/3cupfresh blackberries (frozen will also work; do not thaw first)
1and 1/2 tablespoons fresh lime juice
2sticks (8 ounces) unsalted butter, VERY soft
4and 1/2 cups confectioners sugar (more only if needed), sifted
Fresh berries, for decorating (optional)
For the Triple Berry Cupcakes:
Preheat oven to 350 degrees (F). Line 2 cupcake tins with paper liners; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, beat the butter, vanilla, and sugar on medium speed until light and fluffy, about 2 minutes. Add in the eggs, one at a time, beating well after each addition. On low speed, beat in the sour cream, then the milk. Turn mixer off. Add in 3 and 1/4 cups of the cake flour, baking powder, baking soda, and salt. Use a fork to lightly toss the dry ingredients together. Using a rubber spatula, gently fold the dry ingredients into the batter, mixing just until combined. Be sure not to over mix here or you cupcakes will be dense! In a separate bowl combine the berries and the remaining flour; toss to coat, being sure all of the berries have been dusted in the flour. Add the berries to the batter and, using a rubber spatula, gently fold them into the batter, stirring just until combined.
Spoon batter into prepared cupcake tin (filling 1/2 way each - not more or they'll overflow) and bake for 18 minutes, or until a toothpick inserted in the center comes out clean. Cool cupcakes 10 minutes in the pan, then carefully transfer them to a cooling rack to cool completely.
For the Triple Berry Frosting:
Combine the berries and lime juice in a small saucepan; bring to a simmer over medium heat. Once simmering, stir the mixture occasionally, breaking up the berries with a wooden spoon, until they've become soft enough to mash effortlessly. It should look like thick berry jam. Once the mixture can coat a spoon (you should have 1/4 cup of berry puree) remove from heat. Allow mixture to cool completely before using. <--- That part is SUPER important! You can place the puree in the fridge to speed this process up.
When ready to make buttercream:
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld mixer, beat the butter on medium-speed (scraping down the sides as needed) until completely smooth; about 2 minutes. Add in the confectioners sugar, one cup at a time, on low speed. Once all of the sugar has been added, add in the berry puree and beat until completely combined; about 1 minute. Add in the salt. Once all of the ingredients have been added, increase the mixer speed to medium-high and beat frosting for a full 2 minutes - this thickens up the frosting and makes it nice and fluffy. If the frosting appears too thin, you may add a little more confectioners' sugar; if the frosting appears too thick, you may add a teaspoon or two of milk. Frost cooled cupcakes as desired and top with fresh berries, if using.
Triple berry puree may be made up to three days ahead and stored in the fridge, covered, until needed. Triple berry puree must be completely cooled before use. Cupcakes are best eaten the day they are made but will keep, covered and stored in the fridge, for 3 days. Bring back to room temperature before serving. You will have enough batter to make 16 cupcakes.