Preheat the oven. Preheat the oven to 350°F. Line an 8-inch square baking pan with aluminum foil or parchment paper, leaving enough overhang to lift for easy removal. Set aside.
Make the crust. Warm the butter until partially melted in the microwave on high, about 30 seconds. In a medium bowl, combine the softened butter, granulated sugar, vanilla, and salt. Add the flour and cut the mixture with two forks or a pastry cutter just until combined. Press the mixture into the prepared baking dish in one even layer. Par-bake for 15 minutes.
Make the apple filling. In a medium bowl, combine the apple slices, flour, granulated sugar, cinnamon, and allspice/nutmeg. Toss to coat. Set aside.
Make the streusel. In a medium bowl, combine the oats, brown sugar, cinnamon, and flour. Use two forks or a pastry cutter to cut the chilled butter in until the mixture starts to come together in sticky clumps. Set aside.
Assemble the layers and bake. Spread the apples evenly over the par-baked crust and sprinkle the streusel mixture over the top. Bake until the streusel topping darkens, about 40 minutes. Allow to cool for about 20 minutes then lift the foil/parchment overhang and transfer to a cutting board. Slice into squares (16 smaller bars or 12 larger bars). Just before serving, drizzle with warmed sea salted caramel sauce.
Notes
Make Ahead & Freezing Instructions: The bars will stay fresh stored in a sealed container in the refrigerator for up to 4 days. You can wrap and freeze the bars for up to 3 months; frozen bars may be thawed overnight in the refrigerator before serving.
Apples: Just like my caramel apple pie, I like to include at least one Granny Smith apple for tartness. You can use a combination of Pink Lady or other sweet apple varieties. I recommend 3 medium apples, but you just want around 3-4 cups once sliced.
Larger Baking Pan: Recipe can easily work in a 10-inch by 7-inch rectangular dish as-is. However, if you plan to bake in a larger dish, I suggest doubling the recipe (minus the salt) and baking for up to 55 minutes.