Heat the oven to 180C/fan 160C/gas 4. Lightly butter a 900g loaf tin and line with baking paper. Put the butter, sugar and orange zest in a large bowl, and beat with an electric whisk for 5 minutes until light and fluffy. Add one egg, and beat until fully combined before adding the second. Briefly mix in the vanilla. In a separate bowl, mix together the flour, baking powder and ¼ tsp of salt. Add the flour mixture to the wet ingredients in two additions, alternating with the yogurt. Add the poppy seeds, then grate the marzipan directly into the batter. Mix to combine.