Directions
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Make the shortbread: Arrange the rack in the middle of the oven and heat to 350°F. Place the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment. (Alternatively, use an electric hand mixer and large bowl.) Beat the butter and sugar together on medium speed until light and fluffy, 3 to 5 minutes. Add the flour and beat on low speed until crumbly, making sure not to over-mix the dough.
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Transfer to an 8x11-inch baking dish. Bake until light golden brown, 18 to 20 minutes. Place on a wire rack.
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Make the toffee: Place the butter, sugar, condensed milk, and maple syrup in a medium saucepan and bring to a boil over medium heat, stirring constantly. Continue to boil, stirring, until thickened and golden brown (but not burned), 5 to 7 minutes. Pour over the shortbread layer. (If you love caramel flavor, use all of the sauce, or use less for a thinner layer.) Let sit until set, at least 1 hour or overnight.
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Melt the chocolate chips in a small saucepan over low heat. Pour over the cooled caramel layer and spread into an even layer. Sprinkle with sea salt (if using), and cool completely. Cut into 12 pieces before serving.