Preheat oven to 350°F (180°C). Spray a 13x9-inch baking pan with baking spray with flour. Line pan with parchment paper, letting excess extend over sides of pan.

In a large bowl, place chopped chocolate and butter.

In a medium saucepan, whisk together granulated sugar, 1½ cups (360 grams) water, brown sugar, and milk; bring to a boil over medium-high heat, stirring occasionally to make sure sugar dissolves. Whisk in cocoa. Pour sugar mixture over chocolate mixture, and let stand for 2 minutes. Stir until chocolate and butter are melted and mixture is combined. Whisk in sour cream, eggs, and vanilla.

In a medium bowl, whisk together flour, baking powder, salt, and baking soda. Gradually add flour mixture to chocolate mixture, whisking just until combined. Pour batter into prepared pan.

Bake until a wooden pick inserted in center comes out clean, 45 to 50 minutes. Let cool completely in pan.

Using excess parchment as handles, remove cooled cake from pan. Spread Whipped Chocolate Frosting on top as desired. Garnish with chocolate curls and chocolate eggs, if desired. Store in an airtight container for up to 3 days.