Line a cupcake tin with muffin liners and preheat your oven to 175℃ / 350℉ (no fan).

In a large bowl, cream the butter and sugar together until light and fluffy. Add the room-temperature eggs one at a time, whipping well after each addition, followed by the vanilla extract.

Sift the flour, baking powder, and salt together for your dry ingredients. Lower the mixer speed and gradually add the dry ingredients in thirds, alternating with the room-temperature milk. Mix until just combined—avoid overmixing.

In a small bowl, combine the vinegar and baking soda. The mixture will bubble up, so immediately fold it into the batter until, just combined.

Spoon or pipe the batter evenly into the cupcake tin, filling each cavity about ¾ full.

Bake for 20-22 minutes at 175℃ / 350℉, or until a toothpick inserted into the center comes out clean. Be sure not to overbake to avoid drying them out. Allow the cupcakes to cool completely on a cooling rack before filling and frosting.

Cook the strawberry pieces, lemon juice, and sugar together over medium heat for about 7 minutes. In a separate bowl, mix the cornstarch with water until smooth. Add a tablespoon of the warm strawberry sauce to the cornstarch mixture, then gradually pour the cornstarch mixture into the strawberry-sugar mixture. Stir constantly and cook for another minute until the sauce thickens.

Start by softening your butter. Next, set up your stand mixer with the paddle attachment or grab your electric hand mixer, depending on what you're using.

Cream the softened butter on medium-high speed for 3 minutes until it becomes light in color and fluffy in texture.

Gradually add the powdered sugar, one tablespoon at a time, mixing on low speed. Once all the sugar is incorporated, increase the speed to medium-high and continue to whip for about 4 minutes, or until the mixture is light, fluffy, and has increased in volume.

Next, with the mixer on low speed, add the lemon juice, heavy cream, vanilla extract, strawberry reduction, and salt. Continue to whip for another minute until the frosting is smooth and silky.

The strawberry frosting should be perfectly fluffy and pipeable. If it’s too thick, add more cream, one tablespoon at a time. If it’s too thin, gradually add powdered sugar, two tablespoons at a time, until the desired consistency is reached.

Remember that the frosting has the best texture right after it’s made when the butter is at room temperature and the mixture is freshly whipped. I recommend using the frosting immediately or storing it in a piping bag at room temperature for a few hours. If refrigerating, allow it to come back to room temperature before use, and you may need to whip it again for a minute or two to restore its volume.

Once the cupcakes have cooled to room temperature, make a small hole in the center of each one and fill it with about one tablespoon of strawberry sauce.

Pipe the frosting onto each cupcake, then decorate as desired, e.g., with fresh strawberries.

Serve the cupcakes fresh. Any leftovers can be stored covered and refrigerated for 1-2 days. I recommend serving the cupcakes at room temperature for the buttercream frosting to soften.