Preheat the oven to 350°F. Heat 1/2 cup of granulated sugar in a 10 1/2-inch cast-iron skillet. Cook until deep amber, about 5 minutes. Remove the pan from the heat. Add 4 tablespoons (1/2 stick) butter, whisking vigorously. Spread caramel evenly to coat the bottom of the skillet, and sprinkle with dark-brown sugar. Arrange pineapple slices and almonds on top of sugar mixture.
In a bowl, cream softened butter, 2/3 cup granulated sugar and vanilla bean seeds. Add eggs one at a time, mixing well after each addition, then mix in sour cream. In another bowl, mix the flour, baking powder, baking soda and salt together. Slowly add the flour mixture to the butter-sugar mixture, mixing until fluffy.
Pour batter into pineapple-lined skillet, spreading evenly. Bake for about 40 minutes, until the cake is deep golden and cake tester inserted in center of cake comes out clean. Run a knife around the edge of the cake, invert it onto a plate and remove the skillet. Replace any pineapple and almonds that may have stuck to the pan. Serve warm or at room temperature.
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