In a mixing bowl, combine unsweetened cocoa powder, corn starch, and salt. Whisk in heavy cream and egg yolks until smooth. In a saucepan, heat milk and brown sugar over medium heat until simmering, stirring occasionally. Slowly temper the egg mixture by adding hot milk gradually while whisking continuously. Return to heat, whisking until thickened and bubbling, about 5-7 minutes. Remove from heat, stir in vanilla extract and chopped bittersweet chocolate, then cool in the refrigerator.

Preheat your oven to 350°F (175°C) and grease a 9-inch pie dish with butter. In a microwave-safe bowl, melt together semi-sweet chocolate and salted butter, stirring until smooth. In a large bowl, mix the melted chocolate with sugar, salt, eggs, and vanilla until well-combined. Gently fold in all-purpose flour until just incorporated. Spread the brownie batter evenly in the pie dish and bake for 20 minutes, or until a toothpick comes out with moist crumbs. Allow to cool completely.

In a cold mixing bowl, combine heavy cream, confectioners sugar, and vanilla extract. Using a hand mixer or stand mixer, beat the mixture on medium-high speed until stiff peaks form, about 3-5 minutes. Ensure to stop once you achieve a fluffy texture, avoiding over-beating.

Once your brownie crust is completely cool, pour the chilled chocolate pudding over the crust, spreading it into an even layer. Spoon the freshly whipped cream over the pudding, using a spatula to smooth it gently or create swirls for visual appeal. Top your pie with chocolate curls made from a bar of semi-sweet chocolate.

Cover the assembled Brownie-Bottom Chocolate Pudding Pie with plastic wrap or a lid, and refrigerate for at least one hour. Before serving, consider adding extra chocolate curls or fresh berries for a pop of color.