Well, I have an extremely simple way of softening that butter right up so it's perfectly beatable, creamable and mixable. And before you stop reading, thinking you know what to do... STEP AWAY FROM THE MICROWAVE! Microwaving your butter will merely melt the edges and ruin everything. Yes, EVERYTHING! Then your cake won't work and you will sit on the kitchen floor, crying. Sad times.
So here's what you should do instead:
1. Boil the kettle and then fill a large glass bowl with the resulting hot water. Leave it for a few minutes until the bowl is really warm.
2. Pour the water out of the bowl and then place the bowl, upturned, over your butter. Leave it there for about five minutes (if you're using a standard 250g size block, that is.)
3. Once the butter is partially-softened, use a knife to cut it into chunks, then place it back under the hot bowl for a few more minutes, et voilà - equally and perfectly softened butter for baking. CAKE TIME!