What temperature and time should I use for a cupcake recipe?
Firstly, it strikes me as odd that your recipe has no raising agent - no baking powder, no bicarbonate/baking soda, no self-raising flour. Unless you're whipping a lot of air into the batter, the cakes will barely rise, and you will end up with 'cookies'.

I would add 2 teaspoons of baking powder and 1/4 teaspoon of bicarbonate of soda (baking soda) and see if that helps.

As for the temperature of your oven, I wouldn't go so far as to halve the temperature and double the time. The idea is that you need enough heat for the cakes to rise at a decent rate then form a crust on the outside. 100 degrees will do little but slowly dry the cakes out.

Domestic ovens are rarely well calibrated, so your oven may be running hotter than the dial indicates. Invest in an oven thermometer to make sure you are setting the temperature correctly: I have had an oven run 40 degrees hotter than the dial said before now!

If you haven't got the time to get a thermometer, try setting the oven about 20 degrees cooler on the dial, to around 180 degrees.