Preheat oven to 325°F (165°C). Wrap the outside of a 9-inch springform pan in heavy-duty foil to prepare for a water bath.

In a bowl, combine the Biscoff cookie crumbs, melted butter, and salt (if using) until the mixture resembles wet sand.

Press the crust mixture firmly and evenly into the bottom and slightly up the sides of the springform pan. Bake the crust for 8–10 minutes. Remove and let cool slightly.

To make the caramel sauce: Heat 1 cup granulated sugar in a heavy-bottom saucepan over medium heat, stirring occasionally, until it melts and turns a deep amber color.

Carefully whisk in the cubed butter until fully melted. Slowly stream in the warmed cream while whisking. Let bubble 1–2 minutes, remove from heat, stir in flake salt, and allow caramel to cool to room temperature.

In a stand mixer, beat the cream cheese on medium speed until completely smooth, scraping the bowl as needed.

Add 1 cup granulated sugar and beat until light and creamy. Mix in the sour cream and vanilla until smooth, taking care not to incorporate too much air.

Add the eggs one at a time, mixing on low just until each egg is incorporated. Do not overbeat once eggs are added.

Fold in 1/4 cup of the cooled salted caramel sauce gently until incorporated into the batter.

Pour the cheesecake batter over the baked crust and tap the pan gently to release any large air bubbles.

Place the wrapped springform pan in a larger roasting pan. Pour hot water into the roasting pan until it reaches about halfway up the sides of the springform pan to create a water bath.

Bake at 325°F (165°C) for about 70–80 minutes, until the edges are set but the center still has a slight jiggle. Turn off the oven, crack the door, and let the cheesecake cool in the oven for 45–60 minutes.

Remove the cheesecake from the water bath, run a knife around the edge, and cool to room temperature. Chill for at least 4 hours or overnight until fully set.

Before serving, gently warm remaining salted caramel if needed and spoon over the top of the chilled cheesecake. Finish with extra flake salt to taste.