Make the sticky caramel. In a small saucepan, melt the caramel with butter and bring to a simmer. Cook for about 3 minutes then remove from heat and let cool.
Meanwhile, make the filling. Beat the butter with an electric mixer until white and fluffy. Add the icing sugar and cocoa powder, mix until combined completely. Add the milk and vanilla, mix until fluffy. Add the digestive pieces and combine with a spatula.
Place the chocolate buttercream on a large piece on clingfilm and roll to make a sausage shape the size of your loaf tin. Refrigerate.
Place a piece of thick paper at the bottom of a loaf tin, then cover with parchment paper. Pour some melted chocolate on the side, then spread some cooled down caramel. Place the buttercream "sausage" on the caramel then freeze for 30 minutes.
Cover with a layer of melted chocolate and freeze for another 30 minutes.
Turn the bar upside down on 2 small glasses and cover with a layer of melted chocolate. Let cool and enjoy!