Line a 8x8 inch baking pan with parchment paper; set aside.
In a small bowl, dissolve coffee powder in water (it will be thick).
In a heatproof bowl over a pot of barely simmering water, melt together the white chocolate, bittersweet chocolate, sweetened condensed milk, and coffee mixture, stirring constantly until smooth, about 5 minutes.
Scrape chocolate mixture into prepared pan, smoothing the top. Refrigerate for at least 3 hours, or overnight, until fudge is firm.
Lift fudge out of pan (by grabbing the parchment paper) and place on a cutting board. Cut into squares using a hot knife.
Recipe by Port and Fin at https://portandfin.com/vietnamese-coffee-fudge/