Combine chopped strawberries, sugar, zest, and lemon juice in a pot.

Bring to a boil and cook on medium heat until the strawberries begin to break down and release their juices.

Immersion blend or mash with a potato masher. You can also use a traditional blender or food processor (carefully!).

Once the strawberry mixture thickens, combine cornstarch and 2-3 teaspoons of water in a small bowl. Gradually whisk the cornstarch slurry into the jam for a smooth consistency. The mixture should thicken immediately upon addition. Reduce heat to medium-low if in danger of burning and boil for 60 seconds to cook the cornstarch.

Pour into a bowl or onto a parchment paper lined baking sheet to cool quickly. Cool completely before preparing the cake. This step can be made up to a week in advance!

Preheat the oven to 325°F convection (fan) or 350°F conventional. And place a liner in 21 cupcake tins (this will leave 3 empty in the pan). Cupcake liners are optional, but do make things easier.

In a large bowl, sift together cake flour, baking powder, and salt. Set aside.

In a medium bowl, whisk or stir together milk, vanilla extract, and cooled strawberry jam from above. This will reduce the amount of folding needed to combine the ingredients later and will keep the cake lighter and more tender.

In the bowl of a stand mixer fitted with the whisk attachment, begin whisking the egg whites on high speed until stiff peaks form.

Remove the meringue from the mixer bowl and transfer it to a clean bowl (or switch to a second mixer bowl if you have one! Lucky!).

Don’t stress about any meringue that remains behind. Add the butter and sugar to the bowl. Switch to the paddle attachment and cream the butter and sugar until light and fluffy.

With the mixer on low, alternately add the flour mixture and the milk mixture. I never allow the flour mixture to mix in completely before adding the next round. This helps prevent overmixing.

Remove the mixer bowl from the machine and scrape down the paddle attachment. Fold in any remaining dry ingredients.

Take about ¼ of the meringue and aggressively fold it into the batter. This is called “sacrificing” because you are sacrificing the aeration in the meringue to loosen the batter. Fold in the remaining meringue in 2-3 additions, being careful not to overfold.

Divide the batter between the prepared cupcake tins. Bake in the preheated oven for 14-18 minutes or until a cake tester or toothpick comes out from the center with only clinging crumbs.

Allow cupcakes to cool on a wire rack completely before frosting.

Beat the butter in the bowl of a stand mixer fitted with the paddle attachment until lightened and soft.

Slowly add the powdered sugar, followed by the vanilla and salt.

Once the buttercream has come together, add the strawberry jam 2 tablespoons at a time until the desired flavor and consistency is reached.

Transfer the strawberry buttercream to a piping bag fitted with a large round tip.

Pipe one thick swirl of buttercream on each cupcake. Using a small spoon, create a little divot in the center of each cupcake. Add some remaining strawberry jam and diced strawberries to each cupcake.