In a small dish, dissolve 1 Tbsp sugar in lukewarm water. Sprinkle yeast over the surface. Do not stir. Let stand for 15 minutes.
In a large bowl, mix together 1/2 cup sugar & lemon zest with fingers. Add flour, salt & cinnamon & mix well. Add prepared yeast along with melted butter, egg & egg white. Combine with a fork then add blueberries & citron.
On a lightly floured surface, knead dough for about 10 minutes. If dough is quite sticky add a LITTLE more flour.
Cover & allow to sit in a draft-free, warm place for 30-40 minutes.
Line pan with parchment paper or butter a 7×11-inch baking dish. Divide the dough into 12 equal parts & form into balls. Place them in the prepared pan & let rise for about an hour or until doubled in size.
Whisk together egg wash & lightly brush over buns before baking.
Before buns are finished rising, preheat oven to 350 F. Bake buns for about 30-35 minutes.
Remove from pan & let rest on a wire rack for about 15-20 minutes before piping on the glaze crosses.
Glaze
Mix together powdered sugar, milk & vanilla (if using) until smooth. You will want this to be quite a stiff glaze so only add enough milk to bring it to a thick consistency. Pipe crosses onto the tops of the slightly warm buns. A Ziploc bag with a corner snipped off works well for this small job.