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Lightly grease a 26 x 36cm baking tray with a substantial rim and line with baking parchment. Heat oven to 180C/160C fan/gas 4.
To make the filling, put all the ingredients in a small saucepan and bring to the boil over a medium heat, whisking to dissolve the sugar and prevent lumps. Pour into a small container, cover with cling film and place in the fridge for a few hrs or until fully chilled.
Put the eggs and caster sugar in a large bowl and use an electric whisk to whisk for 5-8 mins or until the mixture has tripled in volume and holds a ribbon on the surface when the beaters are lifted from the bowl. In a separate bowl, mix together the cocoa powder, oat flour and a pinch of salt. Working in three additions, carefully sieve the dry ingredients over the egg mixture and carefully fold in, keeping the mixture light and airy. Carefully pour the batter onto the prepared tray and gently level out to an even layer.
Bake for 12-14 mins or until the cake springs back when lightly pressed. Remove from the oven and carefully slip the cake and parchment onto a wire rack to cool (unlike other roll cakes, this one isn’t pre-rolled as it is more likely to crack).
Once the cake is at room temperature, confidently turn over in one motion onto a piece of parchment sprinkled lightly with caster sugar, then peel off the lining parchment.
Whisk the spiced cream filling until it holds soft peaks, then spread to an even layer, leaving 2cm along the short edge clear. Spoon the dulce de leche randomly all over the filling and use a knife or a spoon to swirl together.
Carefully roll the cake, using the parchment underneath to help, making sure the uncovered sponge edge is at the end of the roll. Don’t worry if the cake splits a little, this is to be expected. Dust the cake with a thin layer of icing sugar and carefully lift onto a serving plate. To finish, drizzle the cake with the white chocolate. As this cake is made with very little fat, it is best served on the day it is made, as it will dry out more quickly.