Preheat oven to 160°C. Grease a 22cm (9in) baba cake pan well; dust with flour, shake out excess.
Melt butter in a small saucepan over medium heat; stir in brown sugar, cinnamon and walnuts. Cool.
Combine milk and coffee in a small bowl; stir until coffee dissolves.
Beat extra butter and caster sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time. Stir in sifted flours, then milk mixture. Spread one-third of the cake mixture into base of pan; sprinkle with one-third of the walnut mixture. Top with remaining cake mixture.
Bake cake for 55 minutes. Leave in pan for 5 minutes before turning onto a wire rack over an oven tray. Cool
Make toffee drizzle. Stir sugar and the water in a small saucepan over medium heat, without boiling, until sugar dissolves; bring to the boil. Reduce heat; simmer, without stirring, until caramel in colour. Add cream; stir for 1 minute or until mixture is thickened slightly.
Working quickly, drizzle toffee on top of cake; top with remaining walnut mixture, pressing down lightly.