Line a large slice pan with baking paper
Melt butter, cocoa, and sugar together in a small saucepan, stirring to dissolve the sugar
Remove from the heat, add vanilla and cool until just warm
Place biscuits into food processor and pulse until chopped
Lightly whisk eggs, stir into chocolate. Mix well!
Combine with biscuits and press into your prepared slice tin
Refrigerate
Prepare icing and pour over slice. Refrigerate overnight