Both butter and sweetened condensed milk need to be at room temperature before preparing the frosting.
In a large bowl mix butter until light and fluffy, for about 5-7 minutes.
Incorporate vanilla extract and coffee powder. Gradually add sweetened condensed milk in thirds, mixing after each addition. Once incorporated the buttercream is ready to use.
Coat the cake with a thin layer of buttercream. Refrigerate for about 15-30 minutes.
Spread a thin layer of buttercream around the middle of the cake and apply coffee beans and dark mini chocolate.
Refrigerate to set for 15 mins.
Cover the top and sides of the cake with the remaining buttercream leaving visible the decorated middle part.
Smooth the surface.
Refrigerate for about 30 minutes to set.
Brush the fault line with a thin layer of edible gold paint.
Remove the parchment paper from the chocolate sail and place it on top of the cake. Fill the sail with coffee beans and brush the chocolate sail with golden paint.