Place the orange zest, orange juice, egg yolks, egg, sugar and salt in a small saucepan and whisk to combine. Bring to a simmer over medium-high heat, whisking constantly, until the sugar dissolves and the mixture thickens, 3 to 4 minutes.
Immediately transfer the curd to a blender and blend on medium speed. With the blender running, add the butter a few pieces at a time and blend until fully incorporated, about 30 seconds.
Transfer the curd to a medium bowl. Set the bowl inside of a large bowl filled about halfway with ice water and whisk constantly until chilled, 3 to 4 minutes. Refrigerate the curd while you whip the cream.
Wash out and dry the blender carafe. Add the cream and blend until stiff peaks form, about 30 seconds. Gently fold the whipped cream into the chilled curd until just combined. Divide the mousse between four 6-ounce ramekins and chill until set, at least 3 hours and up to overnight. Sprinkle the pretzels over the mousse and serve.
If using a blender without a feed hole in the lid, add the butter in 2 batches, blending after each addition until fully incorporated.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.