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Heat the oven to 180C/160C fan/gas 4. Line a 20cm square baking tin with baking parchment. Beat the butter and sugars together in a bowl until pale using an electric whisk.
Stir the baking powder into the spelt flour, then gradually mix into the sugar and butter. Stir in the oats.
For the filling, melt the butter, sugar and 80g of the dark chocolate and all of the milk chocolate in a heatproof bowl set over a pan of simmering water, ensuring the bowl doesn’t touch the water and stirring occasionally. Or, do this in short 20-second bursts in the microwave, stirring between each burst. Remove from the heat, then add the cocoa, spelt flour and baking powder, and mix until well combined.
Lightly dust a sheet of baking parchment the same size as the tin with flour, then spread half of the flapjack mixture over it. Place another 20cm sheet of parchment on top. Roll the mixture out to the edges of the parchment, then peel off the parchment and place in the lined baking tin.
Spread the chocolate filling evenly over the top. Repeat step 4 with the other half of the flapjack mix, and place on top of the chocolate layer.
Bake on the middle shelf for 45-50 mins or until golden brown. Leave to cool completely.
Melt the reserved 20g chocolate in a heatproof bowl over a pan of simmering water or in 10-second bursts in the microwave. Drizzle over the traybake, leave to set for 10 mins, then cut into squares. Will keep for up to three days in an airtight container.