Millionaires' Shortbread

Name: Millionaires' Shortbread
Cook time: 3 h 25 min
Total time: 3 h 25 min
Cuisine: American
Ingredients: 1/2 c. confectioners’ sugar; 1 1/4 c. (2 1/2 sticks) unsalted butter, at room temperature, divided; 1/2 c. finely chopped, toasted slivered almonds; 2 c. all-purpose flour, spooned and leveled; 1 tsp. kosher salt, divided; 1 (14-ounce) can sweetened condensed milk; 1 c. packed dark brown sugar; 1/2 c. pure honey; 3/4 c. heavy cream, divided; 2 tsp. pure vanilla extract; 8 oz. bittersweet chocolate, chopped; Flaky sea salt (such as Maldon), for garnish