Add cream to a small saucepan and finely grate the tonka bean over it. Give it a few stirs and gently heat until just under a simmer - do not let it boil. Remove from the heat and let infuse for 15 minutes. Strain the cream and return to the saucepan.
Gently reheat the cream to just under a simmer - do not let it boil - then reduce the heat to low and add the chopped chocolate to the cream. Stir constantly until the chocolate is just about melted and a few smallish lumps remain. Remove from the heat and stir the chocolate until it is smooth and the lumps have melted.
Stir in the rum until it is completely combined and the chocolate is silky and smooth. Line an approximate 17 x 11.5cm dish with parchment paper (make sure some of the paper overhangs for easy removal later) and pour the chocolate into it. Place into the refrigerator to set for at least 4 hours.
Remove the dish from the refrigerator and pull out the chocolate using the overhanging parchment paper. Peel away the paper from the chocolate.
Warm a sharp, straight knife under running water from the tap and dry completely. Trim off the messy edges from the chocolate (only if you want all neat edges, otherwise, don't worry about trimming the edges). Cut the remaining rectangle of chocolate into 3cm squares (or whatever size you'd like).
Once you've made all the cuts, dust some cocoa powder over the squares and serve within an hour or two for maximum flavor.
Store leftovers in the refrigerator, but know the subtle flavor of the rum and tonka bean will diminish overnight.