Cake – How much white chocolate should I use in a white chocolate cake

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.

The purpose mixing the butter and sugar until creamy is to distribute air through the batter that will help leaven the cake in the oven. Skipping this step can create a denser cake. You want to add the air into the batter before adding the flour in order to prevent over mixing, which creates gluten; causing a tougher texture.

Beating the eggs until foamy and folding them into the batter or adding the eggs to the butter/sugar and beating until fluffy will also create a lighter texture.

The sugar does not need to dissolve in the butter, but it should be evenly distributed. This site gives a good visual on how creamed butter & sugar should look.