Preheat the oven to 425°F. Pour the sweetened condensed milk into a 9x13-inch glass baking dish. Sprinkle the flake sea salt evenly over the top.

Cover the dish tightly with foil, making sure the foil does not extend too far over the edges. Place the baking dish into a larger roasting pan.

Carefully pour hot water into the roasting pan until it reaches approximately one-third of the way up the sides of the glass baking dish.

Bake for 1 1/2 to 2 hours, stirring every 30 minutes, until the condensed milk is thickened and deep golden brown. Check the water level each time you stir and add additional boiling water if necessary.

Remove the pans from the oven. Carefully lift the glass baking dish out of the roasting pan and set it on a heat-safe surface. Allow the caramel to cool for about 5 minutes, then dry the outside of the dish and transfer the caramel to a heat-resistant bowl. Let it cool completely to room temperature.

In a medium bowl, combine the graham cracker crumbs, granulated sugar, brown sugar, and kosher salt. Mix well.

Pour the melted butter over the crumb mixture and stir until all the crumbs are evenly coated.

Transfer the crust mixture to a springform pan. Press it firmly into an even layer across the bottom and nearly all the way up the sides of the pan using the bottom of a measuring cup or glass.

Place the crust in the freezer while preparing the remaining components.

In the bowl of a stand mixer fitted with the whisk attachment, whip the cold heavy cream on medium-high speed until stiff peaks form. Transfer the whipped cream to a separate bowl and set aside.

Replace the whisk attachment with the paddle attachment. In the same mixing bowl, beat the cream cheese, granulated sugar, and powdered sugar on medium speed until smooth and creamy, about 3 minutes.

Add the sour cream, lemon juice, and vanilla extract. Beat for an additional 2 minutes, or until the mixture is very smooth.

Reduce the mixer speed to low and add the whipped cream. Mix for about 10 seconds, then finish folding it in gently by hand with a rubber spatula until fully combined.

Remove the crust from the freezer and pour the cooled caramel over the crust. Spread it into an even layer with an offset spatula.

Cover and refrigerate for 30 minutes.

After chilling, spread the cheesecake layer over the caramel, smoothing the top evenly. Leave about 1/4 inch of space below the top edge of the crust.

Cover carefully, making sure the covering does not touch the cheesecake surface, and refrigerate for at least 8 hours or up to 48 hours before serving.

Remove the cheesecake from the springform pan. Drizzle with caramel sauce and sprinkle with English toffee bits, if desired. Slice and serve chilled.