Preheat the oven to 180 degrees C. Grease a 25cm springform cake tin and line it with greased greaseproof paper.
Break up the chocolate and place it in a heatproof bowl with the margarine. Set the bowl over a saucepan of almost boiling water, making sure the water does not touch the base of the bowl. Leave to melt, then remove from the heat and stir the mixture until smooth.
Meanwhile, place the eggs and sugar in a large bowl and beat with an electric mixer until the mixture has increased considerably in volume and leaves a trail on the surface when the beaters are lifted out. (If using a whisk or egg-beater, set the bowl over a saucepan of almost boiling water, making sure the water is not touching the base of the bowl.)
Add the chocolate mixture and fold in with a large metal spoon. Gradually sift the flour over the top of the mixture, folding it in until it is just combined.
Turn the mixture into the prepared cake tin, gently spreading it to the edges to level the surface. Bake for 15–20 minutes or until the top of the cake feels just firm to the touch. Leave to cool in the tin.
Remove the cake from the tin and peel away the greaseproof paper. Cut into thin wedges for serving, decorating each with a strawberry, if desired. Dust with the sifted icing sugar and cocoa powder.