Why do my homemade chocolate bars start to melt as soon as I take them out of the fridge?
I think the problem is that you don't have chocolate bars, but chocolate mixed with butter, which has a lower melting point (just like buttercream cake icing).

If it's your dentist who is advising you to go slow on chocolate, you might like to try switching away from high sugar 'candy bar' chocolate and onto higher quality chocolate with a cocoa content of 70% and over. The high-quality Swiss brand Lindt, for example, lists the sugar content of their milk chocolate as 23g per serving, but their 70% dark chocolate as 12g per serving and in their 85% super-dark as only 5g of sugar per serving.