Preheat oven to 300F and line a baking sheet with parchment paper.
Beat butter, sugar and oil together until light and fluffy.
Add in the flour, baking soda, salt and vanilla and mix well.
Fold lemon balm leaves into the batter.
Roll dough into small balls (a rounded teaspoon is a nice size) then roll in sugar and place on baking sheet, leaving a little room between the cookies.
Bake for 12-15 minutes, until just slightly golden. Remove from cookie sheet and allow to cool on wire racks.