Whisk custard powder, icing sugar and milk in a small saucepan. Slowly whisk in extra milk. Place over medium heat. Cook, stirring constantly, for 6 minutes or until custard bubbles and thickens. Transfer to a heatproof bowl. Cover surface with plastic wrap. Cool 20 minutes. Refrigerate until cold.
Preheat oven to 180C/160C fan-forced. Grease base and sides of a 23cm round (base) springform pan.
Brush 1 filo sheet with butter. Top with another sheet. Brush with butter. Fold in half and place in base of prepared pan, tucking sides to create a border. Brush another filo sheet with butter, then fold in a pleated pattern at 2cm intervals. Starting from the centre of the tin, coil folded pastry around centre. Repeat with remaining butter and filo until you have filled the pan.
Place custard into a snap-lock bag. Snip off 1 corner. Pipe custard between coils of pastry. Bake for 35 minutes or until pastry is golden. browns. Cool for 5 minutes.
Place walnuts in a small frying pan over medium-high heat. Cook, stirring occasionally, for 3 minutes or until toasted. Add honey. Cook, stirring, for 1 minute or until honey coats walnuts.
Drizzle tart with walnut mixture. Dust tart with cinnamon sugar. Serve.
