Special equipment: Six 4- to 6-ounce ramekins
Spray six 4- to 6-ounce ramekins with cooking spray. Stir together the sour cream, Dijon, horseradish and 1 teaspoon salt in a small bowl. Heat the half-and-half in a small saucepan over medium-high heat until it just barely comes to a simmer.
Meanwhile, put the gelatin in a large bowl, add 3 tablespoons warm water and stir until completely dissolved. Gradually pour in the warm half-and-half while whisking constantly.
Add a splash of the half-and-half mixture to the sour cream mixture and stir to loosen, then add the sour cream mixture to the large bowl with the half-and-half mixture, whisking gently until thoroughly combined.
Pour the mixture into the ramekins (the ramekins will not be full) and refrigerate until set, at least 4 hours. The panna cottas can be kept refrigerated, covered with plastic wrap, up to 2 days
When ready to serve, run a thin knife along the edge of each ramekin and invert the pana cottas onto plates. Sprinkle with the chives and serve immediately.