Preheat the oven to 350°F. Line a large cast-iron skillet (about 10 inches) with aluminum foil, leaving extra foil hanging over the edges for easy removal. Lightly grease the foil.
Melt the butter in a medium saucepan over low heat or in a microwave-safe bowl. Remove from heat and stir in the sugar until well blended.
Add the eggs, one at a time, whisking well after each addition until the mixture is smooth.
Combine the dry ingredients: In a separate bowl, sift together the flour, baking powder, and salt.
Mix the batter: Add the dry ingredients to the butter mixture. Stir just until combined. Add the almond extract and stir again until the batter is smooth.
Assemble: Pour the batter into the prepared skillet. Spread evenly. Sprinkle the slivered almonds evenly over the top, then lightly sprinkle with about 1 tablespoon of the powdered sugar (reserve the rest for after baking).
Bake for 30–40 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Cool and serve: Let the cake cool in the skillet for about 10 minutes. Use the foil overhang to lift the cake out of the pan. Dust with the remaining powdered sugar before slicing and serving.