Preheat your oven to 350℉ and prep a 9x13 pan (grease and line with parchment paper so at least 1 inch is hanging over the edges)
To make the shortbread:In a stand mixer with a paddle attachment (or using a hand mixer) cream the butter and sugar together for 5 minutes.
Add in the salt, vanilla extract, and flour and beat for another 3 minutes until it comes together.
Spread dough out in the baking pan (can use a spatula or your fingers).
Bake for 20-30 minutes until the edges are nicely golden.
To make the filling:Combine the dark brown sugar, corn syrup, maple syrup, eggs, melted butter. vanilla, and kosher salt in a mixing bowl. Whisk together.
Add in the chopped pecans and mix in until just combined.
When the shortbread is done baking, pour the pecan mixture on top and bake for an additional 28-32 minutes until deeply golden.
Sprinkle with flaky sea salt when it comes right out of the oven.
Allow to cool completely to room temp before cutting into it.