Preheat the oven to 350°F. Lightly grease a 9×13-inch baking pan or line it with parchment paper.
Whisk the melted butter and granulated sugar together in a large bowl. Add the eggs one at a time, then stir in the vanilla.
In a separate bowl, whisk together the cocoa powder, flour, salt, and baking powder.
Stir the dry ingredients into the wet ingredients just until combined.
Spread the batter into the prepared baking pan. Bake for 25 to 30 minutes, until the center is set and a toothpick comes out with a few moist crumbs.
Cool the brownies completely before cutting them into approximately 1-inch cubes.
Whisk the instant pudding mixes with 4 cups of cold whole milk for about 2 minutes. Refrigerate for 5 to 10 minutes to thicken.
Set aside ¼ cup of mini chocolate chips. Chop half of the semisweet chocolate bar and shave the remaining half into curls.
Place half of the brownie cubes in the bottom of a 3½- to 4-quart trifle bowl.
Spread half of the chocolate pudding over the brownie layer.
Spread one 8-ounce container of whipped topping over the pudding. Sprinkle with about ⅜ cup mini chocolate chips and half of the chopped chocolate.
Repeat with the remaining brownies, remaining chocolate pudding, and second container of whipped topping.
Garnish with the reserved mini chocolate chips, remaining chopped chocolate, and chocolate curls.
Drizzle with room-temperature hot fudge sauce, if desired.
Cover and refrigerate for at least 2 hours before serving.