Grease and line a 20 cm (8 inch) square cake tin with nonstick baking paper.
Chop half the chocolate into small pieces and set aside. Roughly chop the remainder and put in a heatproof bowl with the butter. Rest the bowl over a saucepan of gently simmering water and leave until melted.
Beat together the eggs, sugar and vanilla bean paste in a separate bowl. Beat in the melted chocolate mixture. Add the flour, almonds and chopped chocolate and stir well. Turn into the tin and spread into the corners.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for about 25 minutes, or until the surface is golden and the centre feels just firm to the touch. Leave to cool completely before removing from the tin and cutting into small squares.