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Oil a 20 x 20cm baking tin and line the base and sides with baking parchment. Heat the oven to 180C/ 160C fan/gas 4. Combine the oil, milk, vinegar, vanilla and yogurt in a jug. Mix the sugar, flour, cocoa powder, baking powder, bicarb and a pinch of salt together in a bowl.
Pour the wet ingredients into the dry and mix until there are no pockets of flour remaining. Tip the mixture into prepared tin and level the surface with a spatula. Bake for 35 mins, or until a skewer inserted into the middle of the cake comes out clean. If any wet cake mixture clings to the skewer, return the cake to the oven for another 5 mins, then check again. Remove from the oven and leave to cool completely in the tin.
Set half of the cookies aside for decorating later. Bash the rest with the end of a rolling pin or blitz in a food processor to chunky rubble. Beat the spread, vanilla and icing sugar together using an electric whisk until fluffy, then gently fold in the crushed cookies until combined.
Put the cake on a board. Spread over the icing. Halve or crumble the rest of the cookies and use these to decorate. Cut into squares. Will keep in an airtight container for four days.