Pre-heat the oven to 350.
Line a 9x9 inch pan with parchment paper and set it aside.
Mix together the butter and icing sugar in a medium bowl.
Mix in the flour with your hands until well combined.
Press into the pan, making sure to press it down very well.
Bake for 5 minutes and then remove it from the oven.
While the crust bakes, make the filling.
In a medium bowl add in the brown sugar and butter and mix well.
Whisk in the eggs, vinegar and then the vanilla.
Add in the raisins and mix.
Pour over the crust and gently spread out the raisins evenly.
Bake for 25-27 minutes, or until the filling is set and doesn't jiggle anymore when the pan is lightly shook.
Cool in the pan on a wire rack until it completely cools.
Once completely cooled, remove from the pan and slice into desired sized pieces.
DO NOT SLICE WHEN HOT.