1. To make Masa: Combine masa harina, salt, and baking powder in bowl. Add 2 1/4 cups hot water, and beat on medium speed until mixture is crumbly. Increase speed to medium-high, and alternately add shortening and broth 1 Tbs. at a time. Beat 15 minutes, or until mixture is light and fluffy. (To test texture, drop 1 Tbs. masa into cold water. If it floats, it’s light enough.) Set aside.
2. To make Filling: Preheat oven to broil. Place chiles on baking sheet, and broil 8 to 10 minutes, or until blackened on all sides, turning occasionally. Place in bowl, cover with lid, and steam 15 minutes. Pull off charred skin by hand, and dip chiles in water to remove blackened bits. Remove stems, seeds, and veins, and cut chiles into 1/2-inch dice.
3. Meanwhile, heat oil in skillet over medium heat. Add onion, and cook 10 minutes, or until browned. Add raisins, broth, oregano, and chopped poblanos; simmer 5 minutes. Remove from heat, and stir in olives. Cool.
4. To assemble tamales: Hold each banana leaf square briefly over low gas burner to soften. Place each banana leaf square shiny side up, atop 10-inch foil square. Spread 1/2 cup Masa in thin layer over center third of each banana leaf. Top with heaping 1 Tbs. Filling. Fold over sides and then ends of banana leaf to enclose filling in Masa, then wrap in foil square.
5. Stack tamales in steamer basket, and steam over simmering water 1 hour, or until Masa is just set and pulling away from banana leaves.
6. Meanwhile, to make Romesco Sauce: Purée peppers, sherry vinegar, and paprika in blender until smooth. Add almonds, breadcrumbs, and garlic, and purée until smooth. With blender motor running, add olive oil in steady stream until smooth sauce forms.
7. To serve: Remove and discard foil from tamales. Serve with Romesco Sauce.