Put the plain flour, fast action yeast, salt and sugar in a large bowl. Make sure that the salt and the yeast do not directly touch each other. Stir together
In a pan on a low heat, warm up the milk and butter until the butter melts, then add the vanilla extract and lemon zest. Let the mixture cool for 5-10 minutes or until lukewarm, then whisk the egg into it
Pour the milk mixture into the flour mixture and bring everything together into a sticky dough
Knead the dough for 10 minutes either by hand on a lightly oiled surface or in a mixer with the dough hook attachment
Place the kneaded dough in a oiled bowl covered with cling film and leave in a warm place to rise for 1 hour or until doubled in size
Roll out the risen dough on a lightly floured surface, shape it into a rectangle measuring roughly 40 x 30cm (16 x 12")
Spread the lemon curd over the top and sprinkle over the white chocolate chips
Roll the dough into a sausage shape from the longest side, trim the ends then cut it into equal pieces. You can mark out the pieces before you cut with a ruler, you want them about 1 inch thick
Place the cut rolls into a lined traybake tin (I used a 12" x 9" one), cover them loosely with cling film and leave them to rise for 30 minutes
Pre-heat your oven to 180C Fan/200C/350F/Gas Mark 4
Bake the rolls for 20-25 minutes, until golden brown. Leave to cool
Drizzle or pipe the melted white chocolate all over the rolls, add the sprinkles if using
Serve immediately, store leftovers in an airtight container for up to 2 days. The buns can be also be frozen if wrapped well