BROWN SUGAR SHORTBREAD

1. Preheat oven to 350° F. Spread the hazelnuts in one layer on a baking sheet and place on the middle rack of the oven. Toast for 10-15 minutes, until the hazelnuts give off a nutty aroma and have browned slightly. The skins should start to blister. Allow to cool slightly, then wrap nuts in a clean kitchen towel. Rub nuts in towel to remove loose skins (don't worry about skins that don't come off) and cool completely. Coarsely chop half of the hazelnuts. 
2. Line a 9 inch square cake pan with parchment paper. 
3. Whisk together flour and salt. 
4. With the paddle attachment of a stand mixer, cream the butter and brown sugar together on medium-high speed until super light and fluffy, about 5-7 minutes. Add the vanilla and mix until blended. Be sure to stop the mixer and scrape down the sides of the bowl a few times.
5. Add the dry ingredients and mix just until incorporated, scraping down the sides of the bowl.
6. Lightly press the crumbly dough evenly into the lined cake pan.
7. Bake for 22-24 minutes until the edges are golden brown and the top is firm to the touch. Set aside to cool.
8. Layer the chocolate chips onto the baked Brown Sugar Shortbread Crust, then layer the hazelnuts on top. Carefully and evenly pour the Hazelnut Chocolate Maple Rum Filling over the top. Bake for another 18-20 minutes until the filling has set.
9. Cool completely and allow to set for at least 1-2 hours before cutting into a 4 x 4 grid to make 16 total squares. Sprinkle liberally with sea salt flakes.

HAZELNUT CHOCOLATE MAPLE RUM FILLING

1. Combine the brown sugar and maple syrup in a heavy saucepan over medium heat. Cook, whisking occasionally, until the sugar has completely dissolved. Remove from heat and add the butter, rum, and vanilla.
2. Allow the mixture to cool, then add the eggs and whisk until smooth. (Do not add the eggs while the syrup is hot or they will begin to cook.)