Pre-heat the oven to 180C.
Prepare your cake tins. I baked this in 4 lined 7" tins. If you only have 2 tins, just bake the batter in 2. It will take a little longer to cook, then slice each layer into 2 when cool.
Cream your butter and both sugars until pale, light and fluffy.
Add your eggs one at a time, beating well in between each addition. (Don't worry if it looks a little curdled.
In a separate bowl, sift together your flour, cocoa powder and baking powder. (I never used to listen when a recipe told me to do this, please do it! I promise it's worth it as it means the baking powder is really evenly distributed through the cake.)
Gently stir in your flour to the rest of the batter until just combined.
Finally, stir in your yoghurt and vanilla extract.
Share between either 2 or 4 tins. If using 4 tins and you want to make each one even, there should be about 375g of batter in each tin.
Don't make this batter and leave it hanging around before putting into the oven. The baking powder will begin to react straight away, so whip it into the oven quickly.
Bake for about 20-25 minutes in 4 tins, or 35-40 minutes in 2 tins. This will really vary a lot though depending on your oven. So just check regularly towards the end of the cooking time. As soon as the cake 'pulls' away from the side of the tin, is springy to the touch and a skewer comes out clean, it's done.