Preheat oven to 350 degrees. Line a muffin tin with cupcake liners; set aside.

On a piece of parchment paper, sift together the flour, baking powder and baking soda.

In a large bowl, beat the eggs, yolks, and sugar together until pale yellow. Add the butter, vanilla, lemon zest, juice and buttermilk to the mixture; beat until well incorporated.

Gradually add the dry ingredients to the batter, mixing until smooth. Divide batter evenly among the prepared muffin cups and bake for about 18 minutes, or until a toothpick inserted in the center of one comes out clean. Remove cupcakes and allow to cool on a wire rack for 15 minutes before filling and frosting.

To fill cupcakes: Using a small paring knife, insert the tip of the knife into the cupcake at about a 45 degree angle halfway between the center and outer edge of the cupcake. With the knife at this angle, cut a circle into the top of the cupcake - you should have just cut a small cone out of the center of the cupcake. Cut the bottom of the cone off and discard it, leaving a small "lid". Repeat on all cupcakes.

Spoon a very small amount of jam (between 1/2 to 1 teaspoon) into the cavity in each cupcake. Put the "lid" back on the cupcake. Frost cupcakes as usual.

With an electric mixer, beat butter in a medium bowl until creamy. Add in 2 cups of the powdered sugar and beat until well incorporated. Add the milk, vanilla, and strawberry jam and mix until well-combined. Add in the last cup of powdered sugar, a small amount at a time, until the frosting reaching your desired spreading or piping consistency. Spread or pipe onto cupcakes. Will last, in a covered container in the refrigerator, for up to 10 days.