Pre-heat the oven to 180C, setting it to top/bottom heat function. Place the chocolate, the butter and the Fleur de Sel salt in a heat resistant dish, and melt it above a bubbling water bath, stirring it a few times. Make sure you do not overheat the mixture! Add the sugar, and mix it together with the dollop at low heat, in 30 seconds.
Remove the dish from the water bath, and mix the eggs into the chocolate dollop one after the other. Add the flour and the pear balsamic vinegar as well.
Fill the chocolate dollop into the prepared muffin baking forms, and bake them in an oven for 14-15 minutes. Serve the tartlets with poached or steamed pears or with pear compote. Decorate them with thyme.