Preheat the oven to 425 degrees F with rack in the middle. Cut plums in half and remove the pits. Set plum halves on a plate. Brush the inside of a 10" cast iron skillet or 10" deep-dish pie plate with the butter.
Place eggs in a medium bowl and whisk until the whites and yolks are blended. Whisk in the milk, sugar, vanilla, spices and orange zest, juice and liqueur. Add flour and whisk until a smooth batter forms.
Pour the batter into the pan. Set in the plums, cut side down.
Sprinkle the top of the clafouti with the almonds. Bake 25–27 minutes, or until a golden crust has formed and a toothpick comes out clean when inserted and removed from the middle of the clafouti. (Clafouti will puff on the sides as it bakes, but will settle down once it cools.) Cool on a rack 20 minutes, and then dust with icing sugar.
Cut into wedges and serve.
Note: If you don’t wish to use orange liqueur, simply replace it with 2 Tbsp. more orange juice.