Line an approximately 28 × 18cm brownie tin or similar-sized dish with baking paper.
Break the white chocolate into smaller pieces and melt using either a bain-marie or microwave. Bain-marie: Place the chocolate in a heatproof bowl over a saucepan of gently simmering water, making sure the bottom of the bowl doesn’t touch the water. Stir gently until smooth. Microwave: Heat the chocolate in 15-second bursts, stirring thoroughly between each one, until just melted and smooth. Cool the chocolate down for a few minutes before adding to the rest of the ingredients. You do this by stirring the chocolate gently. Don't let it set too hard though.
Add the marshmallows, chopped lollies and pistachios, if using, to a large bowl.
Pour over the melted white chocolate and gently fold everything together until evenly coated.
Transfer the mixture to the prepared tin and spread it into an even layer. Scatter festive sprinkles over the top.
Refrigerate for 3–4 hours, or until completely firm.
Once set, remove from the tin and cut into approximately 15–18 pieces. Serve chilled, or allow it to soften slightly at room temperature if preferred.