To make the custard, combine custard powder and sugar in a small saucepan; gradually stir in the milk. Stir over heat until the mixture boils and thickens. Remove from heat; stir in the butter and essence. Press plastic wrap over surface; cool
Preheat the oven to 180°C. Grease a deep 22cm round cake pan; line with baking paper.
Beat butter and brown sugar in a medium bowl with an electric mixer until light and fluffy. Add eggs, one at a time, beating until combined between each addition. Stir in the sifted flour and buttermilk, in two batches.
Whisk the cooled custard until smooth. Spread half of the cake mixture into the prepared pan; top with custard. Spread the remaining cake mixture over custard until completely covered. Overlap apple slices on top; brush with the extra melted butter, then sprinkle with combined caster sugar and cinnamon.
Bake for about 1 hour or until firm; stand 5 minutes before turning onto a wire rack to cool.