Heat the oven to 180C/160C fan/gas 4. Butter a 20cm square baking tin and line with baking parchment. Tip the oats into a heatproof bowl. Melt the butter, sugar, golden syrup, maple syrup (if using) and a pinch of sea salt together in a pan set over a low heat. When the sugar has dissolved after a few minutes, stir well and bubble briefly, then pour over the oats and stir to fully coat. Scrape the mixture into the tin and press down into an even layer using a spatula. Bake for 20 mins until just starting to brown. Leave to cool slightly.

For the caramel layer, warm the condensed milk, butter and sugar in a saucepan over a low heat, stirring with a spatula until the butter has melted. Turn the heat up to medium and simmer for 8-10 mins, stirring continuously and scraping the base until the mixture has thickened and turned light brown – when it’s ready, the texture will change and a clear line should show when you run your spatula along the base of the pan. Pour the caramel over the oat base and smooth the surface. Leave to set at room temperature for 1 hr.

Melt the dark chocolate in a heatproof bowl over a pan of simmering water, ensuring the bowl doesn’t touch the water. Or, do this in the microwave in short bursts. Repeat this in a separate bowl to melt the white chocolate, if using. Pour the melted dark chocolate over the caramel layer, tilting the tin back and forth so the chocolate evenly covers the caramel. Drizzle over the white chocolate (if using), and use a skewer to swirl the two together to create a marbled effect. Leave to set at room temperature for 2-3 hrs for a soft-set chocolate layer, or chill for 1 hr if you prefer it to snap. Once set, lift the flapjacks from the tin using the baking parchment, and cut into squares. Will keep in an airtight container for up to four days.