Set up the bain-marie by adding about 1-2cm of boiling water to a small saucepan, and placing a small glass bowl on top. The bottom of the glass bowl should not touch the water underneath.
Partially fill a sink or empty washing up bowl with cold water, ready to place the bowl of melted chocolate in to cool down
Break ¾ of the dark chocolate bar into chunks and place into the glass bowl over the bain-marie. On a medium-low heat, wait until the chocolate is fully melted while stirring occasionally. Then continue to heat until the chocolate has reached exactly 46°C or 114.8°F. Remove the bowl from pan immediately and place into the prepared sink of cool water. Add the remaining ¼ of unmelted chocolate into the bowl and stir until the thermometer is reading 26°C or 78.8°F.
As soon as this temperature has been reached, place the bowl back into the bain-marie on a medium heat, and wait until the chocolate has heated up to 32°C or 89.6°F, then immediately remove from the saucepan. Do not let the chocolate get hotter than this - if it does, you will need to repeat the previous steps by cooling back down and reheating again.
Pour the tempered chocolate into the silicone mould and spread it out to each corner using the end of a flat knife or spoon. Gently tap the mould to evenly disperse the chocolate.
Quickly prepare the white chocolate, if using as a topping. I don't bother tempering it as you won't be using very much of it. Simply break up into chunks and microwave in a heat proof plastic or ceramic bowl until melted, then use a small spoon to drizzle it over the dark chocolate base. If you like, you can get fancy using a cocktail stick to drag vertical lines through the chocolate to create extra nice patterns.
Finish by sprinkling over your other toppings. You can opt for chopped nuts, dried fruit, or other fun toppings like toffee chunks.
Leave the chocolate bars at room temperature to cool for about 45 minutes, then transfer to the fridge and continue to cool until fully set (approx 2 hours in total). Once ready, enjoy immediately or wrap up each individual bar in cellophane as a gift.