Preheat oven to 350 degrees F and place 8 (8 ounce) glass jars on a baking sheet
Seperate the egg yolk and egg whites, place the egg yolks in the mixing bowl and set the egg whites aside
In a mixing bowl with whisk attachment, combine egg yolks and ½ cup granulated sugar; whisk until combined pale yellow color and fluffy
Slowly add milk, vegetable oil and vanilla extract until foamy
On low speed, add in flour, baking powder and salt. Pour the egg yolk mixture into another bowl and set aside
In a mixing bowl with whisk attachment, whisk egg whites until stiff peak; Slowly add in the remaining ½ cup granulated sugar to the egg whites
Fold the egg whites into the egg yolk mixture until fully combined
Divide the batter into the 8 glass jars and bake for 20 minutes or until the sponge cake is light golden brown and baked through
Once the cakes are removed from the oven, poke several holes in the top of the cakes; set aside and allow to cool