01 - Preheat the oven to 350°F (175°C). Grease and line a 9x13-inch sheet pan with parchment paper.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and fine sea salt.
03 - In a large bowl, beat the softened unsalted butter and granulated sugar with an electric mixer until the mixture is light and fluffy, approximately 3 minutes.
04 - Beat in the large eggs one at a time, ensuring thorough mixing after each addition. Stir in the pure vanilla extract.
05 - With the mixer on low speed, gradually add the dry ingredients and the whole milk alternately, beginning and ending with the flour mixture. Mix until just combined, avoiding overmixing.
06 - Gently fold the rainbow confetti sprinkles into the batter using a spatula.
07 - Spread the batter evenly into the prepared sheet pan. Bake for 32–36 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely in the pan on a wire rack.
08 - For the buttercream: In a large bowl, beat the softened unsalted butter until creamy. Gradually add the sifted powdered sugar, whole milk, pure vanilla extract, and a pinch of fine sea salt. Beat for 3–4 minutes until the mixture is smooth and fluffy.
09 - Spread a generous layer of the vanilla buttercream over the completely cooled cake.
10 - Decorate the top of the cake with additional rainbow confetti sprinkles and arrange the edible sugar pearls artistically for a Rococo-inspired effect.
11 - Slice the cake and serve.