In a large mug place the chopped dates and water. Place in microwave for 1 minute to soften the dates.
Take the mug out of the microwave, add in the bicarb soda and mash with a fork - it will fizz a bit.
Add the butter to the mixture and microwave again for 10-15 sec until melted.
Finally, add in the sugar, flour and egg yolk and mix to thoroughly combine.
Place in the microwave for a final 1 minute. It may need a further 10 sec to finish it, depending on the strength of your microwave.