01 -
Set oven temperature to 350°F (176°C).
02 -
In a large mixing bowl, beat softened butter and granulated sugar together with a hand mixer 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 them to the mixture.
04 -
Stir in vanilla extract followed by all-purpose flour. Mix with a spoon or spatula until fully combined.
05 -
Break unsweetened and semi-sweet chocolate into ½-ounce pieces and place in microwave-safe bowls. Microwave at 50% power for 2½ to 3 minutes, stirring after 2 minutes, until completely melted. Alternatively, melt chocolate in a double boiler.
06 -
Pour melted chocolate into the batter and stir until thoroughly incorporated.
07 -
Grease a 9x13-inch baking dish using non-stick cooking spray or Wilton cake release for optimal results.
08 -
Transfer the batter into the prepared pan. Bake in the preheated oven for 30 to 35 minutes or until a cake tester inserted in the center emerges slightly moist. Allow to cool completely.
09 -
If desired, spread chocolate frosting evenly over the cooled squares.
10 -
Sprinkle chocolate sprinkles over the frosted squares.
11 -
Cut into 16 equal pieces and serve.