For the brownie base: Preheat the oven to 350 degrees F.
Lightly coat a round 12-inch pizza pan with cooking spray. Top with parchment paper and spray again with cooking spray.
Add the chocolate hazelnut spread and eggs to a bowl and mix until smooth. Fold in the flour until it is all mixed in. Pour the mixture into the center of the pizza pan and, using a spatula, spread to the edges of the pan in an even layer. (The dough will be quite thick.) Bake until the brownie is cooked through and a toothpick inserted in the center comes out clean, 20 to 25 minutes.
Meanwhile, make the hazelnut ganache: Heat the cream in a pan over a medium heat. Place the chocolate hazelnut spread and chocolate chips in a bowl. Pour the hot cream over and whisk until melted and smooth.
For the toppings: Put the almond bark in a microwave-proof bowl and microwave in short bursts of 30 seconds, stirring in between, until it is melted.
Remove the pizza base from the oven and allow to cool completely in the pan. Carefully slide a spatula under the brownie to loosen it from the parchment and slide onto a cake stand or serving platter.
Pour the ganache over and spread, leaving a 1/2-inch border. Decorate with the candy, then drizzle over the melted almond bark, spooning it back and forth in lines. Scatter over the sprinkles.