Prepare the Crust: Preheat the oven to 180°C (350°F). In a mixing bowl, combine the crushed digestive biscuits, melted butter, and brown sugar. Stir until the mixture is evenly combined. Press the mixture into the bottom of a tart pan, spreading it out evenly. Bake for 10-12 minutes or until golden brown. Let the crust cool completely.
Make the Salted Caramel Filling: In a saucepan over medium heat, melt the butter and brown sugar together. Stir constantly until the sugar has dissolved. Once the mixture starts to bubble, add the heavy cream and continue to stir until smooth. Allow the mixture to simmer for 3-4 minutes, then remove from heat. Stir in the sea salt and let it cool for a few minutes.
Assemble the Tart: Pour the salted caramel filling into the cooled crust. Spread it evenly with a spatula.
Add the Chocolate Chips: In a separate bowl, mix the dark chocolate chips and milk chocolate chips. Sprinkle them evenly over the caramel layer. If you prefer a smooth chocolate layer, you can melt the chocolate chips in a microwave or double boiler and then spread it over the caramel layer.
Chill the Tart: Refrigerate the tart for at least 2-3 hours or until the filling has set and the chocolate is firm.