Make the Caramel: Heat sugar in a saucepan over medium heat, stirring until melted and deep amber (6-8 minutes). Add butter, whisk until melted. Slowly pour in warmed cream while whisking. Stir in vanilla and sea salt. Cool in a jar. Stores 2 weeks refrigerated.
Toast the Pecans: Spread on a baking sheet and toast at 325°F for 6-8 minutes until fragrant. Cool and chop. Boosts flavor by at least 40% compared to raw.
Make the Streusel: Whisk flour, sugars, cinnamon, nutmeg, and salt. Pour in cooled melted butter and mix with a fork until clumpy. Fold in pecans. Keep chunks big.
Make the Batter: Preheat oven to 350°F. Cream butter and brown sugar for 3-4 minutes. Add eggs one at a time, then sour cream and vanilla. Alternate adding flour mixture and milk on low speed. Do not overmix.
Assemble: Spread half the batter in the pan. Add half the streusel and drizzle 3-4 tablespoons of caramel. Top with remaining batter, then remaining streusel. Press lightly.
Bake: Bake at 350°F for 45-55 minutes until a toothpick comes out clean. Tent with foil if browning too fast.
Cool and Drizzle: Cool 20-30 minutes in the pan (warm, not hot). Drizzle remaining caramel over top. Sprinkle with flaky sea salt. Slice and serve.