Melt 80g butter and add to crumbs with cocoa. Process to combine.
Press into six 10cm loose-bottomed tart pans.
Chill for 20 minutes until bases are firm.
Place sugar and remaining butter in a pan over low heat, stirring, until butter has melted.
Add condensed milk and stir over low heat for 5 minutes or until mixture is a thick, golden caramel. Remove from heat and then add chocolate and golden syrup, stirring to combine.
Fill tart shells with caramel mixture, smoothing tops with a palette knife. Chill for at least 1 hour to set. Whip cream until soft peaks form.
When ready to serve, place tarts on plates, top with sliced banana and whipped cream, then dust with cocoa.