Roughly chop the mixed nuts.

In a microwave-safe container, mix instant coffee with water, cover loosely with plastic wrap, and heat in a 600W microwave for about 10 seconds.

In a saucepan, melt the butter over medium-low heat, then add the coffee mixture and marshmallows, stirring until melted.

Add the mixed nuts, corn flakes, and chocolate chips to the saucepan and mix thoroughly.

Spread the mixture onto a sheet of parchment paper and refrigerate for about 1 hour until firm.

In a microwave-safe container, break the white chocolate into pieces and heat in a 600W microwave for about 2 minutes, stirring until melted. Drizzle over the chilled mixture and refrigerate for another 30 minutes until set.