Melt 1/2 cup butter in a light-colored saucepan over medium heat. Swirl frequently as it foams and sizzles. After 4 to 5 minutes, golden-brown flecks will appear at the bottom and a rich nutty aroma will rise. Immediately remove from heat and pour into a heatproof bowl to stop cooking.

Preheat oven to 350°F (175°C). Grease a 10-inch cast-iron skillet or a 9x9-inch ceramic baking dish with butter.

In a large bowl, toss the sliced Honeycrisp and Granny Smith apples with brown sugar, maple syrup, lemon juice, flour, cinnamon, nutmeg, and vanilla until every slice is glossy and spiced.

Pour the apples into the prepared skillet, arranging them in a relatively flat, compact layer.

In a separate bowl, stir together the rolled oats, flour, dark brown sugar, granulated sugar, cinnamon, and fine sea salt.

Pour in the warm browned butter and stir with a fork until coarse, clumped crumbs form.

Scatter the brown butter oat streusel generously over the apples, covering them completely edge-to-edge.

Bake at 350°F for 40 to 45 minutes until the streusel is deeply golden and crisp, and the spiced apple juices are thick and bubbling vigorously around the rim.

Let the crisp rest on a wire rack for 10 to 15 minutes before serving so the spiced caramel juices slightly thicken.

Scoop generous warm portions into bowls and top immediately with a giant scoop of melting vanilla bean ice cream and caramel drizzle!