Preheat oven to 350 degrees. Grease and 8×8 pan with butter and line with parchment paper.

To make the cookie base, in a large bowl, using a rubber spatula, mix together butter, brown sugar and granulated sugar until smooth.

Add the eggs and vanilla, mix again until incorporated.

In a separate bowl, gently whisk flour, baking powder, baking soda, salt and cornstarch.

Slowly add the dry ingredients to the butter mixture, mix as you go.

Evenly spread dough into the prepared pan and bake for 34-36 minutes, or until the base is golden brown and no longer jiggles in the middle.

When the cookie base is almost done (about 5 minutes left of baking), work on the toffee layer.

Add water and cornstarch to a small bowl. Mix well with a fork until all cornstarch chunks disappear. Set to the side.

In a saucepan, over medium heat, melt the butter. Add the brown sugar, heavy cream, salt and corn syrup.

Whisk constantly until mix comes to a boil and pour in the cornstarch/water slurry. Stir until the mix thickens, about 1-2 minutes.

Then mix in the vanilla. The mixture should still be runny.

When the cookie base is done baking, pour the hot toffee over the base.

Bake for another 11-12 minutes. Toffee should be bubbling.

Remove from the oven, let the bars rest for about 2 hours.

To melt the dark chocolate, place chocolate chips or chunks in a heat proof bowl, microwave in 10 second intervals, stir and repeat until chocolate is completely melted.

When ready to serve, lift bars from the pan, drizzle with the melted chocolate and sea salt.