Quick and Spicy Cocoa Shrimp

Name: Quick-and-Spicy Cocoa Shrimp
Ingredients: 3 Tbsp. extra-virgin olive oil; 1 medium onion, finely chopped; 4 garlic cloves, finely grated; ⅓ cup double-concentrated tomato paste; 2 Tbsp. chili powder; 1 Tbsp. unsweetened cocoa powder, preferably Dutch-process; 2 tsp. smoked paprika; ½ tsp. cayenne pepper; 2 tsp. all-purpose flour; 1¾ cups low-sodium vegetable broth; Juice of 1 medium lime; 2 Tbsp. dark brown sugar; 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt, plus more; 1½ lb. large shrimp, peeled, deveined; Cilantro leaves (for serving); Tostadas and lime wedges (for serving; optional)