Preheat the oven to 160°C (approximately 325°F). Butter and flour the pan. Beat the butter with the sugar until light and fluffy. Gradually stir in the eggs and lemon zest. Stir for 5 minutes until smooth. Mix the flour with the starch and the baking powder. Alternately add the flour and the milk into the egg mixture. Separate the batter into two bowls. Add cocoa powder and 2-3 tablespoons of milk to the second bowl.
Place half of the white batter into the pan. Cover with the chocolate batter and swirl the chocolate in with a fork to create a marble pattern. Bake in a preheated oven for 45-60 minutes. The cake is done when a toothpick is inserted and comes out clean.
Turn the cake out of the pan and let cool on a wire rack. Mix the powdered sugar with the lemon juice into a stiff glaze and spread on the cake. Allow to dry. To serve, cut the cake into pieces.