Soak raisins in hot water for 10 minutes.

Drain raisins.

Combine drained raisins and softened butter in a medium bowl.

Stir until butter is melted and well combined with raisins.

Add brown sugar, corn syrup, vanilla, and salt to the raisin-butter mixture.

Mix all ingredients thoroughly until smooth.

Fill prepared tart shells 7/8 full with the butter tart filling.

Place filled tart shells on a cookie sheet.

Bake in a preheated oven at 425°F (220°C) for 12-15 minutes, or until the pastry is golden brown and the filling is bubbly.

Remove from oven and cool on the cookie sheet for 30 minutes before serving.