In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Using an electric mixer, beat the butter and granulated sugar in a large bowl until light and fluffy, about 3 minutes.
Beat in the eggs, then the buttermilk and orange zest.
Reduce the mixer speed to low and gradually add the flour mixture, mixing just until incorporated.
Heat oven to 350 degrees F. Coat a 9- by 13-inch baking dish with cooking spray.
Spread half the batter into the prepared baking dish.
Using a slotted spoon to drain out some of the juice, spread 1 cup spiced cranberry mixture over the top, then gently swirl it into the batter. Top with the remaining batter, then dollop with the remaining spiced cranberries.
Bake until the cake is golden brown and a pick inserted into the center comes out clean, 45 to 50 minutes. Let cool for 5 minutes.
While the cake cools, make the icing: In a small bowl, whisk together the confectioners' sugar and milk until smooth. Drizzle the icing over the warm cake and top with sliced almonds. Let cool completely before serving.