Preheat oven to 350°F and fill a cupcake pan with liners.
In a medium bowl, whisk together the flour, cornstarch, baking powder and salt.
In a large bowl using a mixer on medium speed, cream together the softened butter, sugar and lemon zest until light and fluffy. Add the egg, egg white and vanilla extract and mix for another 1-2 minutes.
On low speed, alternately mix the dry ingredients and kefir into the wet ingredients just until combined. Be careful not to over-mix.
Scoop the batter into the cupcake liners (about ⅔-3/4 full) and bake on the middle rack until a toothpick comes out clean (about 16-20 minutes). Let them cool in the pan for 5-10 minutes then transfer to a wire rack.
Strawberry Filling
Pulse the strawberries and sugar in a food processor for 1-2 rapid bursts. Or, finely chop the strawberries and mix them well with the sugar.
Lemon Strawberry Buttercream
In a medium bowl, mix together the softened butter, lemon zest, strawberry filling and powdered sugar until smooth.
Cupcake Assembly
Cut the center out of each completely cooled cupcake (about ⅔ deep; save the cut-out tops without the fluffy insides).Spoon strawberry filling inside but leave a tiny space to put the cupcake top (without the insides) back in place.Pipe frosting on the cupcakes and enjoy!
Notes
Kefir: This recipe is sponsored by my favorite kefir brand, Lifeway. Their organic whole milk plain or lemon kefir is best in this recipe.Strawberries: Fresh berries work best in this recipe.Storage: Store cupcakes in an airtight container in the refrigerator for 4 days. Unfilled and unfrosted cupcakes can be stored in the freezer and then brought to room temperature, filled, frosted and served.Nutrition: The provided nutrition information is an estimate, accuracy is not guaranteed.