• Preheat the oven to 350F degrees and adjust the oven rack to the middle position. Line the cupcake pan with 6 cupcake liners.
• In the mixing bowl of a stand mixer fitted with the paddle attachment, mix the flour, baking powder, salt, and sugar to combine. If flour sprayed up the side of the bowl, scrape it down with a spatula. Note – If using a stand mixer, I recommend the 3-quart mixing bowl (link above) for the small batch batter. Otherwise, you can use a hand mixer and medium mixing bowl.
  2/3 cup all-purpose flour, 6 tablespoons granulated sugar, 1/2 teaspoon baking powder, 1/4 teaspoon salt
• Add the butter cubes to the dry ingredients. Mix on medium-high speed until the mix resembles coarse sand.
  3 tablespoons unsalted butter, room temperature and cut into cubes
• In a small mixing bowl or liquid measuring cup, whisk together the sparkling water, egg, and vanilla. Tip - Make sure it is a fresh can of sparkling water so you get the benefit of the bubbles to make the cupcakes fluffy.
  1 large egg, 1 teaspoon vanilla extract, 1/3 cup sparkling water, berry flavored
• Turn the mixer on low and slowly drizzle the wet ingredients into the dry. Mix for about 15 to 20 seconds, or until no dry streaks remain. Turn the mixer off, scrape down the sides of the bowl, and mix a couple more times with the spatula. Expect a thin batter.
• Pour the batter into the prepared cupcake liners. They will be about 1/2 full. Bake for 15 to 18 minutes, or until a toothpick inserted in the middle comes out clean.
• Cool the cupcakes in the pan for 5 minutes on a wire cooling rack. Then remove from the pan and cool completely on the wire rack.

Berry Cupcake Filling

• While the cupcakes are baking and cooling, make the filling. Add the berries, sugar, water, and lemon juice to the saucepan. If using frozen berries, no need to thaw. Heat the berries over medium-low heat until they start to break down, release their juices, and get jammy. This may take about 8 to 10 minutes. Use a small wooden spoon to help break down the berries for a smoother filling.
  1/3 cup fresh strawberries, chopped, 3 tablespoons fresh blueberries, 2 teaspoon sugar, 2 teaspoons water, 2 teaspoons lemon juice
• Let the berry jam cool completely before filling the cupcakes. It can also be made ahead of time and refrigerated.
• Assemble the cupcakes. Use a small sharp knife and cut out a hole in the center of the cupcakes (eat the cupcake pieces!). Spoon about a tablespoon of jam into the cupcake hole. Frost with your favorite frosting (I give a few options below in the Notes), and top with a few berries for garnish. Tip - Don't cut the center of the cupcake too deep or wide, or they will fall apart!
• Depending on the frosting, the cupcakes can be covered and stored at room temperature for 2 to 3 days. If using a cream cheese base frosting, I recommend refrigerating them.