Place semisweet chocolate chips in a large, microwave safe bowl. Heat chocolate in 20-30 seconds intervals, stirring in between each heating until completely smooth. Add in sugar and corn syrup and mix until combined. Add in rum and mix, be patient with this step it may take a few minutes for rum and chocolate to combine. Add in vanilla, espresso powder & vanilla wafer crumbs and mix thoroughly, add in chopped walnuts and stir until everything is combined.