Heat water and sugar in a medium saucepan over medium heat. Bring to the boil then reduce heat and allow to simmer for 5 minutes. Add the butter and stir until melted.
Turn off heat.
Add the ground biscuits, hazelnuts, almonds, rum and vanilla. Mix until well incorporated. Remove one third of the mixture and place into a mixing bowl. Stir through the 200 g melted chocolate into the one third mixture, incorporating well.
Grease a square 22cm baking tin and line with baking paper. Gently press the chocolate mixture into the base of the lined tin, smoothing the surface and making sure it is evenly filled.
Take the remaining two thirds of the mixture and press it over the top, spreading evenly and smoothing the top.
Melt the 120 g chocolate and whisk in the olive oil. Pour over the biscuit mixture and tip the pan so the chocolate covers the surface evenly. Set aside to cool for 20 minutes, then cover with plastic and refrigerate overnight.
To serve, slice the bajadera into small rectangles, or whatever shape you desire. Dust with cocoa, if you wish.