Prepare the Salted Caramel Sauce: In a medium saucepan over medium heat, melt the granulated sugar, stirring occasionally until it turns an amber color (5-7 minutes). Add the sliced butter, stir until combined, then gradually pour in warmed heavy cream, vanilla extract, and sea salt. Allow to cool.

Make the Crust: Preheat oven to 350°F (175°C). Crush Biscoff cookies into fine crumbs, combine with melted butter, and press into the bottom of a springform pan. Bake for 10 minutes and set aside to cool.

Prepare the Filling: In a mixing bowl, beat softened cream cheese until smooth, then gradually add granulated sugar and vanilla extract. Mix in eggs one at a time, then add heavy cream, sour cream, and cornstarch until combined.

Combine and Bake: Gently fold in cooled salted caramel sauce into the cheesecake filling, then pour over the prepared crust. Bake for 60-70 minutes until the edges are set and the center jiggles slightly.

Chill and Serve: Once cooled, refrigerate for at least 4 hours or overnight. Drizzle with remaining caramel sauce and sprinkle flaky sea salt before serving.