To make the cake:
1) Preheat the oven to 180C/Gas Mark 4. Lightly butter and flour a 23 by 5-cm round cake pan, and set it aside.
2) Sift the flour, baking powder, and salt into a medium bowl, and set it aside.
3) In a standing mixer fitted with the paddle attachment, or with a hand-held electric mixer, cream the butter, sugar, and citrus zest until light and fluffy. Add the eggs, one by one, beating well after each addition. Beat in the vanilla. With the mixer running on low, add the flour mixture in 3 parts, alternating with the milk in 2 parts, until just combined.
4) Pour the batter into the prepared pan. Bake until golden brown and a tooth pick inserted into the center of the cake comes out clean, about 30 minutes. Cool in the pan for 15 minutes on a rack. Turn the cake out of the pan to cool completely.
To make the cream filling:
5) In a small, heavy-bottomed saucepan, heat the milk over medium heat, until bubbles appear around the edges. Remove from the heat.
6) In a medium bowl, whisk together the yolks, sugar, cornflour, and salt. While whisking, gradually pour the milk into the eggs. Pour the egg mixture back into the saucepan. While whisking constantly, bring the mixture to a boil over medium heat; cook for 1 minute or until thickened. Remove from the heat, and stir in the butter and vanilla. Put the custard in a medium bowl, and lay a piece of cling film directly on the surface to prevent a skin from forming. Refrigerate until chilled, about 1 hour. Whip the cream until it holds a soft peak. Lightly whisk the chilled custard to loosen it up. Fold the cream into the custard; refrigerate until ready to assemble the cake.
To make the glaze:
7) Put the chocolate and golden syrup in a small bowl. In a small saucepan, bring the cream to a boil. Pour the cream over the chocolate and stir until smooth. Cool slightly before glazing the cake.
To assemble the cake:
8) Slice the cake horizontally with a serrated knife, into two pieces, starting about 5/8ths of the way up the sides of the cake. (The top piece should be slightly thinner than the bottom.) Place the bottom of the cake on a 23-cm cardboard cake round or flat plate. Spread the cream just to the edges of the cake and cover with the top layer. Pour all of the glaze over the cake. Tilt the cake so the glaze covers it evenly and runs down the sides. Refrigerate until ready to serve.