Preheat oven to 180°C/350°F (160°C fan-forced). Grease and line the base of a 23cm/9″ tart tin with a loose base.
Break biscuits and crush into fine crumbs using a food processor or rolling pin in a ziplock bag.
Transfer crumbs to a bowl, add melted butter, and mix until combined resembling wet sand.
Press mixture firmly into the tart tin base and up the sides. Place tin on tray for easier handling.
Bake for 10 minutes. Remove and let cool; chill for 5 minutes if desired.
For caramel, melt butter in saucepan over medium heat; add brown sugar and whisk until combined.
Once bubbling and sugar dissolves, gradually add condensed milk, whisking constantly until steaming, about 4 minutes.
Remove from heat and stir in salt to taste. Pour caramel over biscuit base.
Bake tart for 15 minutes until caramel edges are golden with a soft center.
Cool tart at room temperature while making ganache.
Place cream and dark chocolate in heatproof bowl and microwave in two 30-second bursts, stirring until smooth.
Let ganache thicken for 10 minutes, then pour over caramel layer and smooth or swirl the top.
Sprinkle with sea salt flakes and refrigerate tart for 1 ½ hours to set chocolate.
Before serving, bring tart to room temperature and carefully remove from base or just the tin sides.