Melt 1 cup of unsalted butter in a medium saucepan or skillet over medium heat. Swirl occasionally as it foams and sizzles for 5 to 6 minutes, until the milk solids turn a rich toasted amber brown and release an intoxicating nutty aroma. Remove from heat immediately and pour into a heat-safe bowl so it doesn't scorch.

Preheat your oven to 350°F (175°C). In a large bowl, toss the sliced Honeycrisp and Granny Smith apples with 1/4 cup of the prepared warm brown butter, dark brown sugar, cinnamon, nutmeg, allspice, lemon juice, vanilla extract, and cornstarch until every slice is glossy and evenly coated. Transfer into a 9x13-inch baking dish or 10-inch deep cast iron skillet.

In the same bowl, combine rolled oats, flour, brown sugar, chopped pecans, cinnamon, and sea salt. Pour in the remaining 3/4 cup of warm brown butter. Use a fork or clean fingertips to toss until large, clumped, crumbly streusel clusters form.

Evenly scatter the oat streusel clusters over the spiced apples, covering the fruit completely. Bake uncovered at 350°F (175°C) for 38 to 42 minutes, until the oat topping is deeply golden-brown and fragrant, and the amber apple juices are bubbling vigorously along the edges.

Allow the apple crisp to cool for 10 minutes so the caramel juices slightly thicken. Spoon warm, generous portions into bowls and top immediately with a large scoop of vanilla bean ice cream and a swirl of salted caramel sauce.