Line a 8x8-inch square baking pan with foil, leaving a couple of inches of overhang. Spray the foil with nonstick baking spray.
In a medium bowl, combine the butter and brown sugar. Beat with an electric mixer on medium-low speed until light and fluffy. Add the flour and salt and beat until thoroughly combined. Transfer the mixture (it will be crumbly) to the prepared pan. Press it evenly into the bottom of the pan.
Transfer the mixture (it will be crumbly) to the prepared pan. Press it evenly into the bottom of the pan. (use the bottom of a coffee mug to help press it in the pan if needed.)
Place the pan in the air-fryer basket. Set the air fryer to 350°F for 13 minutes. When the crust has 5 minutes left to cook, start the filling.
For the Filling
In a medium saucepan, combine the butter, brown sugar, maple syrup, and milk. Bring to a simmer, stirring occasionally. When it begins simmering, cook for 1 minute. Remove from the heat and stir in vanilla and finely chopped pecans.
Carefully pour the filling evenly over the crust, gently spreading with a rubber spatula so the pecans and liquid are evenly distributed.
Set the air fryer for 12 minutes or until mixture is bubbling. (The center should still be slightly jiggly–it will thicken as it cools.)
Remove the pan from the air fryer and sprinkle with the salt. Cool completely on a wire rack. When cooled to room temperature, transfer the pan to the refrigerator. Use the foil overhang to remove from the pan and cut into 8 bars.
Serve at room temperature.
Finely chop the pecans small for easier cutting of finished bars
Chill the bars before serving. It's a must!
Leave extra foil to hang over the sides of the baking dish to create handles to easily remove the cooked tart.
To make this in an oven, try 325F. You will likely have to bake for 3-4 minutes longer than specified in the recipe.