Heat the oven to 180C/350F/gas mark four. Grease a tin about 20x20cm. Put the flour and cornmeal into a bowl with the sugar and salt and mix well, then cut in the butter and mix to form a dough. Press into the tin, prick all over with a fork and bake for about 25-30 minutes, until golden and crisp. Allow to cool.
Put the remaining butter and sugar in a wide, heavy-based pan with the syrup, condensed milk and salt, and heat gently, stirring to melt the butter. Bring to a simmer and continue to simmer for about 10-15 minutes, until thick and fudgey. Pour over the shortbread and smooth with a palette knife. Leave to set.
Melt the chocolate in a heatproof bowl over a pan of simmering water or in a microwave, and spread evenly over the set caramel. Leave until solid, then turn out and cut into squares.