For the Cupcakes:

1. Preheat the oven to 350°F. Line cupcake pans with cupcake liners.
2. In a large mixing bowl, combine the sugar, vegetable oil, eggs, and lemon extract. Beat on medium speed until the eggs and oil are well incorporated, scrape down the bowl as needed. Next, add the lemon zest and the sour cream and beat until well combined.
3. Combine the dry ingredients in a separate bowl. Alternate adding half the dry ingredients followed by half of the liquid ingredients (buttermilk and lemon juice) and mix just until the flour starts to incorporate. Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well mixed.
4. Using a large cookie scoop, divide the batter evenly, filling each liner 2/3 full. Bake at 350°F for 15-18 minutes. Test the cupcakes for doneness by inserting a toothpick into the center of the cupcake. If the toothpick comes out clean, the cupcakes are done. Cool Completely.

For the Blueberry Sauce:

1. In a medium-sized bowl, combine the blueberries and sugar and stir to combine.  Transfer to a large saucepan, add 2 tablespoons water, and cook over medium-high heat until the mixture starts to bubble and release the juices, stirring often. As the blueberries break down, combine 1 tablespoon water with the cornstarch and stir to dissolve; add to the blueberries and mix to combine.
2. Turn down the heat and let simmer to allow the blueberries to soften and further cook down. Cook until sauce starts to thicken; around 10-15 minutes. Stir often so the bottom does not burn. Remove from the heat and cool completely, refrigerate if necessary. The sauce will further thicken as it cools.
3. Separate ¼ cup of the blueberry sauce and puree in a food processor or blender until it reaches a smooth consistency. Set aside.

For the Frosting

1. Cut the butter into pieces. Using the paddle attachment, whip the butter for 5-7 minutes, scraping down the bowl occasionally, and beating until the butter looks light in color.
2. Add 2 cups of powdered sugar and the vanilla extract and mix on low speed until incorporated, then increase the speed to high, and beat for approximately 60 seconds or until well combined.  Scrape down the bowl. Add the remaining 3 cups of powdered sugar, lemon juice, and salt. Beat on low until the ingredients start to mix together. Increase speed to high and beat for another 2-3 minutes until it reaches desired consistency.
3. Separate out half of the frosting and set aside in a bowl. To the mixer add 2 tablespoons of blueberry puree and mix to combine. If the frosting seems thin or runny, add an additional ½ cup of powdered sugar. Add additional puree to taste.

To Assemble

1. Assemble the piping bag. Fit a large piping bag with a large tip and fill it half full with both vanilla and blueberry frosting (see notes for additional tips).
2. Remove the centers using a cupcake corer. Save the tops of the cupcakes to the side.
3. Use a spoon to fill the center of each cupcake with the blueberry filling. Gently push the top of the cupcake back on top of the filling and frost.