01 -
Preheat the oven to 350°F (176°C).
02 -
In a large mixing bowl, cream the unsalted butter with granulated sugar until light and fluffy.
03 -
Add eggs one at a time, beating well after each addition. Crack eggs into a separate bowl first to remove any shell fragments before adding.
04 -
Stir in vanilla extract and all-purpose flour with a spoon or spatula until fully combined.
05 -
Break chocolates into ½-ounce pieces and place in microwave-safe bowls. Microwave at 50% power for 2 ½ to 3 minutes, stirring after 2 minutes, until fully melted. Alternatively, melt over a double boiler.
06 -
Pour the melted chocolate into the batter and stir until completely incorporated.
07 -
Grease a 9x13-inch baking dish using non-stick cooking spray or preferably Wilton cake release.
08 -
Transfer the batter to the prepared pan and bake for 30 to 35 minutes, or until a cake tester inserted comes out slightly moist. Allow to cool.
09 -
If desired, spread chocolate frosting evenly over the cooled squares.
10 -
Sprinkle chocolate sprinkles on top of the frosted squares.
11 -
Cut into 16 equal pieces and serve.