Instructions

Step 1
Preheat oven to 180C/160C fan-forced. Grease a 6cm-deep, 22cm square cake pan. Line base and sides with 2 layers of baking paper. Grease a 6cm-deep, 22cm round (base) springform pan. Line base with baking paper. Line sides with 3 layers of baking paper, extending paper 8cm above edge of pan (see Kim's tips).
Step 2
Prepare cake mix following packet directions (reserve icing sachet for another use). Pour into prepared square pan. Bake for 30 minutes or until a skewer inserted into the centre of cake comes out clean. Stand in pan for 10 minutes. Turn onto a wire rack to cool completely.
Step 3
Using a serrated knife, trim cake to 1 1/2cm high. Using the base of the springform pan as a guide and a small sharp knife, cut around pan to form a circle of cake. Place into base of prepared round pan. Fill any small gaps with cake trimmings. Discard remaining cake trimmings.
Step 4
Using an electric mixer, beat 1/2 cup sweetened condensed milk, 1 1/2 cups thickened cream and lime jelly crystals until just-firm peaks form. Spoon mixture into prepared pan over cake base, levelling top with a spatula. Tap gently on bench to remove any air bubbles. Freeze for 2 hours or until just firm.