Grease sides and line base of a 21cm or 22cm round or square cake pan with greased baking paper.
Mix coffee essence and instant coffee together until granules have dissolved.
Cream butter, sugar and vanilla essence until light and creamy. Beat in eggs, one at a time, beating well after each addition. Beat in milk until well combined.
Sift flour, baking powder and bicarbonate of soda and gently fold into the creamed mixture. Divide mixture into two equal portions. Stir the cocoa powder and coffee essence mixture into one portion of the batter. Mix well. Leave the other portion plain.
Fill prepared pan with alternate spoons of the chocolate-coffee and plain batter. Level surface with a spatula and swirl the batter with chopsticks to get a marbled effect.
Bake in a preheated oven at 170°C for 50-60 minutes or until a skewer inserted into the centre of the cake comes out clean. Leave the cake to cool in the tin for 10-15 minutes before turning out to cool completely on a wire rack.