Set the oven to 190°C/375°F/Gas Mark 5.

Melt the chocolate and butter, either in a bowl in a microwave oven or over a pan of hot water. Lightly beat in the eggs and vanilla extract. Add the sugar and flour to the bowl and stir to combine all of the ingredients. Spoon the mixture into the lined cake tin and level the surface.

Bake in the centre of the oven to 40-45 mins, until just set to the touch in the centre. Remove from the oven and leave to cool in the tin for 5-10 mins. Transfer to a wire rack and leave to cool completely.

Dust brownie with icing sugar. Cut into 24 squares to serve.