Place a 12-inch nonstick skillet on the stove top on medium heat. Melt the butter with sugar and stir until it starts to caramelize, about 15 minutes. Remove from heat and let cool slightly. Preheat the oven to 350 degrees F.
In the same skillet, layer the apples in a circular pattern making sure they are tightly placed and cover the whole surface of the pan. Add another layer of apples and cover them with puff pastry dough folding the dough inside the edges of the pan.
Put the tatin pie in the oven and bake for 30 minutes or until the crust is golden brown.
Remove the tatin pie from the oven and let sit for 5 to 10 minutes before flipping the pie.
For Sponge Toffee: Combine the sugar, syrup, and vanilla in a medium-size saucepan. Stir over medium heat until the sugar is dissolved. Continue to cook to 300 degrees F, on a candy thermometer or hard crack stage, about 10 minutes.
Remove from the heat, using a wooden spoon, stir in the baking soda quickly. It will become frothy. Pour into an 8 by 8-inch/20 by 20-cm pan, lined with parchment paper and buttered.
Let cool for about 30 minutes and break into pieces.
Chocolate Dipped Sponge Toffee: Dip the sponge toffee in chocolate for a homemade crunchy treat.
Sponge Toffee Ice Cream: Mix sponge toffee pieces into prepared ice cream.
Tatin dish can be substituted for a skillet. Golden delicious apples can be substituted by Granny smith or Cortland apples or any other apple that holds when cooked. You can use a candy thermometer or a simple trick: take a spoon and drop a tiny drizzle of the mixture into a bowl of water. If it crystallizes, you've reached the hard crack stage.