In a saucepan off the heat, combine the sugar and cornstarch. Add the eggs, lemon juice and butter, whisking until smooth. Slowly bring to a boil, stirring constantly with a whisk. Pour into a bowl and let cool. Cover and refrigerate until lukewarm or chilled.
Place 1 or 2 scoops of ice cream in a sundae bowl. Drizzle with lemon topping. Using a pastry bag fitted with a star tip, garnish with a rosette of whipped cream. Sprinkle with crumble.