Preheat your oven to 350°F (175°C). Crush the digestive biscuits into fine crumbs and mix with melted unsalted butter. Press the mixture into the bottom of a springform pan. Bake for 10 minutes, then cool completely.

In a large mixing bowl, beat the cream cheese and granulated sugar together until smooth. Add the eggs one at a time, mixing well after each addition. Stir in vanilla extract and pour the filling over the cooled crust.

Bake the cheesecake for 45 minutes, until the edges are set. Turn off the oven, crack the door, and let cool for about 1 hour at room temperature. Refrigerate for at least 4 hours or overnight.

In a saucepan, combine sweetened condensed milk, brown sugar, and unsalted butter over medium-low heat. Stir until it thickens, about 10 minutes. Cool slightly before pouring over the cheesecake.

Chill the cheesecake again for at least 1 hour to set the caramel topping before serving.