Preheat oven to 350℉ and prepare 12 cupcake tins with liners
Whisk the flour, cornmeal, baking powder, baking soda, sugar, and salt in a medium mixing bowl and set it aside.
In a separate mixing bowl, whisk the eggs, egg yolk, sour cream, lemon zest, lemon juice, and vanilla.
Add the dry ingredients to the wet and beat gently to combine. Gradually add the oil, folding it in with a rubber spatula until incorporated and the batter is smooth.
Scrape the batter into the cupcake tins until they’re ⅔-¾ full.
Using a small spoon, dollop 1 teaspoon of jam in the middle of each cupcake and gently swirl it into the batter with a butter knife.
Arrange the strawberries on top of each cupcake - about 4-6 slices per cupcake.
Bake until golden brown and springy on top, about 30-35 minutes.