Cut or crumble the Asda free-from Christmas pudding into small chunks or pieces, and set aside. Mix the ground flaxseed or linseed with 4 tbsp water in a small bowl, and leave to thicken and become gel-like.
Melt 150g of the chocolate, the spread and milk together in a heatproof bowl set over a small pan of just-simmering water (ensure the bottom of the bowl does not touch the water). Stir until the chocolate has melted and the mixture is smooth. Remove from the heat.
Heat the oven to 180C/160C fan/gas 4. Oil a 20cm brownie or square cake tin and line with baking parchment. Briefly whisk the flour, ground almonds, cocoa, sugars and salt together in a large bowl with an electric whisk to break up any lumps of sugar. Make a well in the centre, then whisk in the chocolate mixture, along with the flaxseed mixture and vanilla. Beat until smooth and all the flour is incorporated – the batter should be thick.
Fold in the Christmas pudding pieces and the remaining chocolate chunks until just incorporated, then scrape into the prepared tin. Smooth the surface with a spatula and bake for 40-50 mins until the top has a light crust and there's a slight wobble in the centre. A skewer inserted into the middle should have moist crumbs clinging to it, but no raw batter. Leave to cool completely in the tin.
Cut the brownies into squares and serve at room temperature, or chill for a more fudgy texture. Will keep in an airtight container at room temperature or chilled for up to three days, or frozen for up to three months. Dust with the cinnamon icing sugar just before serving.