Place the digestive biscuits in a sturdy zip-top bag and crush them with a rolling pin until you have fine, even crumbs. Alternatively, pulse them in a food processor. Transfer the crumbs to a bowl and mix in the melted butter (and brown sugar, if using) until the mixture resembles wet sand.
Pour the crumb mixture into a 9-inch loose-bottomed tart pan. Press firmly and evenly into the base and up the sides using the back of a spoon or a measuring cup. Place the crust in the refrigerator to set for at least 30 minutes.
While the base chills, make the filling. In a medium saucepan over medium heat, combine the sweetened condensed milk, brown sugar, and butter. Stir constantly for 5-7 minutes until the butter and sugar have melted completely and the mixture begins to thicken and darken slightly. It should come to a gentle boil – keep stirring! Remove from heat and stir in the vanilla extract and sea salt. Allow it to cool for 10 minutes.
Pour the warm caramel into your chilled biscuit base and spread it evenly. Place the pie back in the refrigerator to chill for at least 2 hours, or until the caramel is completely firm and set.
Just before serving, slice the bananas into ~1/4-inch coins and arrange them in a single layer over the set caramel.
In a large, cold bowl, whip the heavy cream, powdered sugar, and vanilla extract until you achieve firm peaks. Be careful not to over-whip.
Either pipe or dollop the whipped cream over the banana layer. Finish with a generous grating of dark chocolate over the top. Slice, serve immediately, and accept the compliments!
