Pre-heat the oven to 160°C / 140°C fan assisted / gas mark 3 / 320°F.
Line a 23cm cake tin with baking parchment paper and lightly grease.
Melt the chocolate, either in a small bowl set over a pan of simmering water or in the microwave. Set aside to cool.
Place the yoghurt, brown sugar, olive oil, eggs, marmalade and orange zest in a large bowl and beat until smooth using a handheld electric mixer.
Pour the melted chocolate in and stir thoroughly into the rest of the ingredients.
Sift together the flour, cocoa powder, bicarbonate of soda and salt in a separate bowl then add into the mixture and beat until just combined.
Pour into the prepared cake tin. Place in the oven and bake for about 60 minutes.
Remove from the oven and turn out onto a cooling rack.
Melt the extra 2 tablespoons of marmalade in a small saucepan until runny then brush over the top of the cake to glaze.
Leave to cool completely before serving.