Preheat the oven to 350°F and line two 12-cup muffin pans with cupcake liners.

In a medium bowl, whisk together the cake flour, salt, baking powder, baking soda, and lemon zest.

In a separate bowl, whisk together the milk, lemon juice, vegetable oil, and lemon extract.

In the bowl of a stand mixer, beat the butter until smooth. Gradually add the sugar and beat for about 3 minutes, until light and fluffy.

Beat in the eggs, one at a time, mixing well after each addition.

Add the dry ingredients and milk mixture alternately, beginning and ending with the dry ingredients. Mix just until combined—do not overmix.

Fill each cupcake liner about ⅔ full (approximately 3 tablespoons of batter).

Bake for 15–20 minutes, or until a toothpick inserted into the center comes out clean. Cool completely before filling and frosting.

In a small saucepan over medium heat, combine the raspberries, sugar, lemon juice, and 1 tablespoon of the water.

Mash the berries with a potato masher or whisk and bring the mixture to a gentle boil.

In a small bowl, whisk together the remaining 1 tablespoon water and the cornstarch until smooth.

Stir the cornstarch slurry into the raspberry mixture and cook for 1–2 minutes, stirring constantly, until thickened.

Transfer to the refrigerator and cool for about 10 minutes.

Beat the softened butter for 3–5 minutes until smooth and fluffy.

Add the powdered sugar on low speed until mostly incorporated.

Mix in the lemon extract, milk, and salt.

Increase the mixer speed to medium-high and beat for about 2 minutes until light and creamy. Add a splash of milk if the frosting is too thick, or additional powdered sugar if it's too soft.

Once the cupcakes have cooled, use a small knife or cupcake corer to remove the center of each cupcake.

Fill each cavity with about 1 tablespoon of the raspberry filling.

Pipe or spread the lemon buttercream over the top.

Garnish with fresh raspberries or lemon zest, if desired.

Tip: For a shortcut, you can substitute your favorite boxed lemon cake mix and prepare it according to the package directions before filling and frosting as directed above.