Crush the digestive biscuits into fine crumbs—you want them resembling coarse sand rather than large chunks. Mix the crushed biscuits with the melted butter until the mixture resembles wet sand and holds together when pressed. Line an 8-inch springform pan with parchment paper, then firmly press the biscuit mixture into an even layer on the bottom. Chill in the refrigerator while you prepare the remaining components.
Combine the sugar and water in a saucepan over medium-high heat, stirring occasionally until the sugar dissolves. Stop stirring once it begins to boil and let it cook undisturbed until it reaches a deep amber color—about 5-7 minutes. Remove from heat immediately and carefully add the room-temperature whipping cream (it will bubble vigorously). Stir constantly for about 1 minute until the bubbling subsides and the sauce becomes smooth. Add the unsalted butter, salt, and vanilla extract, stirring until fully incorporated. I like to let the caramel cool completely before using it so it reaches the right consistency for folding—this prevents the cream cheese mixture from breaking down. Set aside to cool.
Ensure the cream cheese is softened to room temperature (about 70°F) for a smooth, lump-free filling. In a large bowl, beat the softened cream cheese with the vanilla extract and sifted powdered sugar until completely smooth and creamy, about 2-3 minutes. Gently fold in the cooled caramel sauce from Step 2 using a spatula, being careful not to overwork the mixture. In a separate bowl, whip the cold whipping cream to stiff peaks. Carefully fold the whipped cream into the cream cheese mixture in two additions, folding just until no white streaks remain—overworking will deflate the whipped cream and result in a dense filling. Fold in the chopped Twix pieces gently so they distribute evenly throughout.
Pour the cheesecake filling mixture from Step 3 over the chilled crust from Step 1, spreading it into an even layer. Refrigerate for at least 4-6 hours (or overnight) until the filling is firm enough to support the caramel and chocolate layers. I find that refrigerating overnight gives the best texture—the filling sets properly and creates clean layers when sliced.
Remove the cheesecake from the refrigerator and spread the 1/2 cup salted caramel sauce evenly over the filling, leaving about a 1/2-inch border around the edges (this prevents caramel from running over the sides). Return to the refrigerator for 30 minutes to allow the caramel to set slightly before adding the chocolate layer.
Place the finely chopped semisweet chocolate in a heatproof bowl and pour the heated whipping cream (hot but not boiling) over it. Let it sit for 1-2 minutes, then stir gently until the chocolate is completely melted and smooth. Allow the chocolate mixture to cool for a few minutes until it's warm but not hot, then pour it evenly over the caramel layer. Spread it gently to the edges with a spatula. Return the cheesecake to the refrigerator for at least 30 minutes until the chocolate sets completely.
Once the chocolate layer is completely set, remove the cheesecake from the refrigerator and run a thin knife around the edges before releasing the springform pan. Slice with a hot, damp knife (wiping between each cut) for clean pieces. If desired, garnish with crushed Twix bars, caramel shards, or additional crushed digestive biscuits for texture and visual appeal. Serve chilled.
