Using a fork, mix the cornstarch and water together in a small bowl until the cornstarch has dissolved—mixture is thick. Warm the strawberries and sugar together in a small saucepan over medium heat. Stir the mixture as it cooks, and break up some of the strawberries as you stir. Bring it to a simmer. Once simmering, stir in the cornstarch mixture. Allow to simmer for 5 minutes, stirring constantly. After 5 minutes, remove from heat, stir in the lemon zest, and cool completely. You will have about 1 and 1/4 cup (about 340g) of strawberry filling. The filling thickens as it cools. (It must completely cool, so transferring to a bowl out of the saucepan helps speed that up, as does refrigerating it. You can complete a few other steps in the recipe as the mixture cools.)

Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners. This recipe yields about 15 cupcakes, so line a second muffin pan with 3 cupcake liners, or bake in batches.

In a large bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside. In a medium bowl, whisk the melted butter and sugar together. Mixture will be gritty. Whisk in the egg whites, sour cream, milk, and vanilla bean paste/extract until combined.

Pour the wet ingredients into the dry ingredients and whisk until the batter is completely combined. Batter should be creamy and mostly smooth; a few small lumps are OK.

Pour/spoon the batter into the liners, filling only 2/3 full to avoid spilling over the sides. You should have enough batter for 15 cupcakes.

Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean.

Remove cupcakes from the oven and allow them to cool in the pan for 10 minutes, and then transfer to a wire rack to cool completely. Cupcakes must be completely cooled before filling and topping.

First, set aside 1/2 cup (120g) of the filling for the whipped cream topping. Using a sharp knife, cut a circle into the center of the cooled cupcakes to create a little pocket about 1 inch deep. The piece you removed will be sort of cone-shaped. Spoon some of the cooled and thickened strawberry filling inside each carved-out cupcake—use however much will fit. (I use a teaspoon for this. Usually you can fit between 1–2 teaspoons of filling in each.) Slice/tear off the pointy end of the “cone” piece of cupcake you removed, and gently press the round piece back on top of the filling. See my How to Fill Cupcakes post if you need extra visuals or help with this step.

Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the heavy cream, sugar, and vanilla bean paste/extract on medium-high speed until medium peaks form, about 3–4 minutes. Medium peaks are between soft/loose peaks and stiff peaks, and are the perfect consistency for topping and piping on desserts. If you accidentally over-whip the cream, and it looks curdled and heavy, pour in a little bit more cold heavy cream, and fold it in gently by hand with a spatula until it smooths out. Fold in the reserved 1/2 cup of strawberry filling. Use immediately or cover tightly and chill in the refrigerator for up to 24 hours.

Frost cooled cupcakes. I used an icing spatula, but a knife works just fine. I pile the whipped cream on really high as pictured, and you can certainly do the same! You may have some leftover if you don’t use as much on each cupcake. Garnish cupcakes with sliced strawberries and/or fresh mint.

Serve immediately or store in the refrigerator until ready to serve. Cover and store leftover cupcakes in the refrigerator for up to 5 days. I recommend a cupcake carrier for storing and transporting decorated cupcakes.