In a heatproof medium bowl, whisk together confectioners’ sugar, orange zest (if using), and very hot orange juice until smooth. Add 4 to 6 teaspoons (20 to 30 grams) very hot water, a little at a time, whisking until glaze is thin and smooth.

Heat a medium saucepan of water over low heat until steaming; turn off heat. Immediately place bowl with glaze over saucepan, and gently heat, whisking occasionally, until fluid. (Be careful not to let it get too hot, as it will get syrupy and will crystallize as it cools.) Remove from heat.