Preheat the oven to 170C/325F/Gas 3. Butter a 23cm/9in square cake tin.
Melt the chocolate and butter together, then cool slightly.
Whisk the eggs until thick, then gradually add the sugar and beat until glossy. Beat in the melted chocolate mixture, then gently fold in the flour, baking powder and salt.
Pour just over half the mixture into the prepared cake tin. Scatter over the raspberries, then cover with the remaining mixture.
Bake in the preheated oven for about 40 minutes or until the surface is set. It will be cooked when a skewer into the middle comes out with just a little mixture adhering.
Remove the cake to a wire rack, and allow it to rest for about 20 minutes. Cut the cake into squares and remove them from the tin when cold.