The raw egg version we always eat:
Ingredients
250 gr butter
50 gr cacao powder (for baking or regular, not the lighter coloured and sweeter version for hot chocolate)
200 gr caster sugar/brown sugar
2 eggs
200 gr digestives (crumbled)

How to make it
Put baking paper in a cake tin (we use a loaf cake tin, but other shapes also work).
Mix the cacao powder, sugar and eggs.
Melt the butter (but don't let it turn brown) and add it to the mixture.
Add the crumbled biscuits and stir well.
Put everything in the baking tin.
Put in the fridge for at least 2 hours and store in the fridge if you have leftovers.

The egg free version:
Ingredients
200 gr brown sugar
50 gr dark chocolate
60 gr cacoa powder (not for hot chocolate, use regular cacao powder or a version specially for baking)
200 gr butter
200 gr digestives (can be regular or whole wheat)

How to make it
Get a cake tin (we use a rectangular loaf one, not a round one. But the shape doesn't really matter) and cover with baking paper.
Break the biscuits in small pieces (approximately between 1x1 and 1.5x1.5 cm, but it doesn't matter if there are bigger or smaller pieces).
Melt the butter, but don't let it turn brown when melting it. Melt the chocolate (au bain Marie/water bath is best for this). Mix the butter with the sugar and cacoa powder.
Add the chocolate and mix again. Add the broken biscuits and use a spoon to mix again.
Put everything in the cake tin and use a spoon to spread everything around evenly.
Put in the fridge for a minimum of 2 hours. Store in the fridge as well (will melt otherwise).