Preheat the oven to 350°F and line an 11-by-7-inch baking sheet with parchment paper.

In a large bowl, whisk together the flour, baking powder, baking soda, and salt.

In a separate bowl using an electric mixer, beat the butter until smooth. Add the brown sugar and beat for another minute or so, until the mixture is light and fluffy.

Add the egg and vanilla extract and beat to combine.

Gently fold the dry ingredients into the wet ingredient mixture with a wooden spoon until just combined.

Transfer the cookie dough to the prepared baking sheet and spread it out to an even layer.

Bake for 15 to 17 minutes or until light golden brown.

Remove the baking sheet from the oven and set it on a cooling rack. Let cool for 5 minutes.

While the cookies are cooling, warm the toffee sauce in a saucepan until it is soft enough to be spreadable. Pour the sauce over the cookie bars and tilt slowly in a circular motion until the cookies are completely covered.

Refrigerate for 30 minutes, until the toffee sauce is set.

Meanwhile, melt the chocolate in the microwave in 30- to 45-second intervals, stirring in between, until the chocolate is completely melted and smooth.

Pour the melted chocolate over the toffee cookie bars and spread to an even layer with a spatula.

Sprinkle the toffee bits over the top and then let stand at room temperature for 1 hour.

Once set, sprinkle the coarse salt over the top.

Gently remove the salted toffee cookie bars from the baking sheet, cut into squares with a sharp knife, and serve at room temperature.