How to make a brittler, harder chocolate coating? I'm not sure whether this is a duplicate of this question partly because that question seems to be dealing with making your own chocolate. Which I'm not doing.

In my case I want to add a thin coating of chocolate to flapjacks or homemade biscuits or similar for a sweeter treat. I melt some cooking chocolate then add it to the biscuit base. Then chill in the fridge.

This works fine. However I may want to remove from the fridge, transport the treat to school or workplace, and if left out of the fridge long enough the chocolate can get pretty squidgy.

Is there any trick that'll result in a chocolate coating which stays brittler and harder at room temperature?
You'll want to temper the melted chocolate.  This means holding it at 88° F (31° C) while it cools until it is completely hardened, then finally allowing it to return to room temperature (or lower) afterward.

If you temper the chocolate this way, it will be much harder and last indefinitely at room temperature.

The easiest way to temper chocolate is the "seed method", which means adding a piece of solid (never-melted) chocolate to the fully-melted chocolate after removing it from the heat source and stirring thoroughly until the temperature reaches 88° F / 31° C, and then keeping it at that temperature until you're ready to use it.

(Note that this is for dark chocolate; for temperatures for other types of chocolate and for a more detailed guide to tempering, I suggest you check out the tempering page at Cooking for Engineers.)

The technical explanation for this is that the temperature at which melted chocolate hardens determines its final consistency as a solid.  If chocolate hardens at room temperature, it ends up forming a very weak crystalline structure that begins to melt at even the slightest rise in temperature (i.e. in your hand).  Allowing it to solidify at the highest possible temperature below the actual melting point ensures that it forms a denser crystalline structure and will not melt at room temperature.