Preheat oven to 350F.
In a large bowl, beat the butter, sugars and orange zest until pale and almost fluffy. Beat in the egg and vanilla.
Add the flour, cornstarch, baking soda and salt and stir or beat on low until almost combined; add the chocolate chunks and citron (if you're using it) and stir just until blended.
Drop dough by the spoonful onto a parchment-lined (or buttered) sheet and bake for 10-14 minutes (depending on their size) until golden around the edges but still soft in the middle. Transfer to a wire rack to cool.