Place cardamom pods in an 8" skillet over high heat, and cook, swirling pan occasionally, until charred and blistered in spots, about 4 minutes. Transfer to a bowl, and using a muddler or wooden spoon, crush to expose seeds; then transfer to a 2-qt. saucepan along with cream, ¾ cup milk, ¼ tsp. salt, and vanilla bean with seeds; bring to a simmer over medium-high heat. Remove from heat, and let steep for 10 minutes. Combine remaining milk and gelatin in a small bowl; let sit to soften gelatin, about 5 minutes.
Place white chocolate in a bowl, and pour steeped cream mixture and gelatin mixture over chocolate; let sit for 1 minute. Using a small rubber spatula, slowly stir chocolate from the center until gelatin dissolves, and mixture emulsifies and is smooth. Pour through a fine strainer into a large glass measuring cup; discard solids. Divide mixture among six 6-8-oz. shallow ramekins or custard molds, and refrigerate until set, at least 4 hours.
Meanwhile, whisk together sugar and cornstarch in a 2-qt. saucepan; add juice, liqueur, and zest. Bring to a boil over medium-high heat, and cook, stirring often, until thick, about 3 minutes. Remove from heat, and stir in marmalade and butter until smooth; let cool to room temperature. Add orange segments, and stir gently to combine with sauce; set aside.
To serve, run a paring knife around the edge of each ramekin, invert panna cotta onto a small plate, and spoon orange sauce on the side.