Preheat your oven to 350°F and line a cupcake pan with cute paper liners—your cupcakes are getting dressed up!

In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt. Set it aside like a little flavor treasure chest.

In a large bowl, use a mixer on medium speed to cream the softened butter, sugar, and lemon zest until it’s light, fluffy, and smells amazing. Add in the egg, the egg white, and the vanilla, then keep mixing for another minute or two.

Now lower the speed and gently add the dry mix and kefir in turns—start and end with the dry ingredients. Mix just until everything’s combined. Don’t over-mix; we want soft, tender cupcakes!

Scoop the batter into the cupcake liners, filling each one about ⅔ to ¾ full. Pop the pan onto the middle oven rack and bake for 16–20 minutes, or until a toothpick poked in the center comes out clean.

Let them cool in the pan for 5 to 10 minutes, then move them to a wire rack to cool completely (if you can wait that long!).

Give the strawberries and sugar a quick whirl in a food processor—just 1 or 2 fast pulses will do the trick. No processor? No problem! Just finely chop the strawberries and stir them well with the sugar until juicy and sweet.

In a medium bowl, mix the softened butter, lemon zest, that yummy strawberry mixture, and powdered sugar. Stir it all together until it’s smooth, creamy, and dreamy.

Once your cupcakes are completely cool, carefully cut out the center of each one—go about two-thirds deep. Don’t toss those tops! Just pop out the fluffy insides and save the little “lids” for later.

Spoon that juicy strawberry filling into each cupcake, but leave a little room so you can gently place the top back on like a sweet little cap.

Now for the fun part—pipe on a swirl of frosting, make them look extra cute, and dig in