In a small saucepan, bring ¾ cup (180 grams) water and orange juice to a boil over medium-high heat. Cook, stirring occasionally, until reduced to ½ cup (120 grams), 20 to 25 minutes. Remove from heat, and immediately stir in baking soda and salt. Let stand until slightly warm to the touch, about 15 minutes.
In a large bowl, whisk together molasses, brown sugar, vanilla, zest, and egg. Pour orange juice mixture into molasses mixture in a slow, steady stream, stirring constantly, until combined. Use immediately.
Recipe by Bake from Scratch at https://bakefromscratch.com/shoofly-pie/