Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together flour, baking soda, and salt for the crust.
In a separate large bowl, cream together softened butter and granulated sugar until light and fluffy.
Beat in eggs one at a time, then stir in vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Press half of the dough into the bottom of the prepared baking pan to form the crust.
In a medium bowl, whisk together apple butter, egg, brown sugar, lemon juice, cinnamon, and nutmeg for the filling.
Spread the apple butter filling evenly over the crust.
Crumble the remaining dough over the apple butter filling.
Bake for 40-45 minutes, or until the topping is golden brown and the filling is set.
While the bars are baking, prepare the salted caramel drizzle. In a medium saucepan, combine granulated sugar and water. Cook over medium heat, stirring until the sugar dissolves.
Bring to a boil and cook without stirring until the mixture turns an amber color. Remove from heat.
Carefully whisk in the warm heavy cream (it will bubble up) until smooth. Stir in the butter pieces until melted and incorporated.
Stir in the flaky sea salt.
Let the bars cool completely in the pan before drizzling with salted caramel. Slice and serve.
Notes
These bars are best enjoyed at room temperature. For extra richness, you can add a sprinkle of cinnamon sugar on top of the crumble before baking.