Preheat the oven to 350°F (175°C). Grease a 9-inch tart pan with a removable bottom and set aside.
In a small saucepan over low heat, combine 1/2 cup unsalted butter, 1/2 cup Nutella, and 1/3 cup creamy peanut butter. Stir and heat until melted and smooth.
Remove the pan from heat, add 3 ounces chopped dark chocolate, let sit 1 minute, then whisk until melted and smooth.
Stir in 1/3 cup sugar, 1/4 teaspoon salt, and 1 teaspoon vanilla extract. Whisk to combine.
Beat in the eggs one at a time until each is incorporated, then gently fold in 1/2 cup all-purpose flour until just combined; do not overmix.
Pour the batter into the prepared tart pan and bake 25–30 minutes; the center may still look slightly underdone. Remove from oven and cool completely in the pan.
To make the ganache, place 4 ounces chopped dark chocolate, 2 tablespoons creamy peanut butter, and 2 tablespoons Nutella in a heatproof bowl.
Heat 1/3 cup heavy cream in a small saucepan over low heat until bubbles form around the edges and it is warm, then pour over the chocolate mixture. Let sit 30 seconds, then stir until smooth.
Let the ganache cool slightly until thickened but still spreadable, then spread evenly over the cooled brownie in the tart pan.
Top with assorted candy if desired. Slice and serve once the ganache is set.