Preheat your oven to 375 degrees F and lightly coat the inside of twelve 8-ounce glass jars with non-stick cooking spray.

Whisk the flour, baking powder, and salt together in a medium bowl.

In a separate large bowl, beat the butter and sugar with an electric mixer until the mixture is light and fluffy.

Add the eggs one at a time, beating well after each addition, then stir in the lemon zest and vanilla extract.

Mix in half of the flour mixture, followed by half of the milk and lemon juice, beating until just combined.

Add the remaining flour mixture, then the remaining milk and lemon juice, mixing until the batter is smooth.

Divide the batter evenly among the prepared jars, filling each about one-third of the way full.

Arrange the jars on a baking sheet so they do not touch and bake for 17 minutes, or until a toothpick inserted into the center comes out clean.

While the cakes bake, combine the sugar, cornstarch, and salt for the pudding in a saucepan.

Whisk in the milk and cook over medium-high heat, stirring constantly until the mixture thickens enough to coat the back of a spoon.

Remove the pudding from the heat, stir in the butter and vanilla, and place in the refrigerator to cool.

To make the frosting, beat the heavy cream in a chilled bowl until it begins to thicken, then gradually add the confectioners' sugar and lemon juice while continuing to beat.

Increase the mixer speed to high and beat until the frosting holds soft peaks.

Once the cakes have cooled for 10 minutes, use a spoon to remove the top half of each cake and set it aside.

Spoon a portion of the cooled pudding into the center of each jar, then place the reserved cake top back on.

Finish by topping each jar with the lemon frosting and keep refrigerated until ready to serve.