Make the dough: Cream softened butter and caster sugar together in a large bowl using electric mixer or wooden spoon for 3-4 minutes until light and fluffy and pale yellow in color.
Add flour: Add plain flour all at once. Use your hands to bring the mixture together into a cohesive dough. Knead gently—mixture starts crumbly but comes together with patience. Do not overwork.
First chill: Wrap dough in plastic wrap or cling film and shape into flat disc. Refrigerate for 20-30 minutes until firm and cold to touch.
Prepare baking tray: While dough chills, line large flat baking tray (that fits in your fridge) with parchment paper or greaseproof paper.
Roll dough: Lightly flour work surface and rolling pin. Roll chilled dough to roughly 0.5cm (¼ inch) thick, using even pressure for consistent depth.
Cut hearts: Press heart-shaped cookie cutters firmly into rolled dough. Cut shapes close together. Transfer each heart carefully to prepared baking tray, spacing 1 inch apart. Gather scraps, re-roll once, and cut more hearts for about 10 total.
Second chill: Place baking tray with cut hearts in refrigerator for 30 minutes. This critical chill ensures cookies maintain heart shape during baking.
Preheat oven: While hearts chill, preheat oven to 160°C fan / 180°C conventional (325°F). Position rack in center.
Bake: Bake chilled shortbread hearts for 10-12 minutes until pale golden around edges while centers remain light. Don't overbake—they should look barely done.
Cool on tray: Leave baked shortbread on baking tray for 5 minutes, then transfer carefully to wire cooling rack. Cool completely (at least 30 minutes) before chocolate dipping.
Melt chocolate: Melt milk chocolate and white chocolate separately in small shallow bowls. Use microwave at medium power in 30-second bursts, stirring between intervals, until smooth and fluid.
Dip cookies: Dip bottom half of each cooled shortbread heart into melted chocolate at slight angle. Hold over bowl briefly to let excess drip back. Place on wire rack over parchment paper.
Add sprinkles: Immediately add sprinkles while chocolate is still wet (within 30 seconds). Work with 2-3 cookies at a time for best results.
Set chocolate: Let chocolate set completely at room temperature for about 30 minutes before serving or packaging. Store in airtight container.