Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until well combined. In another bowl, beat the eggs, melted unsalted butter, and milk until smooth. Gradually add this mixture to the dry ingredients, stirring until just combined.

Grease and flour a 9x13-inch baking pan. Pour the batter into the prepared pan, spreading it evenly. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Once the cake has cooled completely, use a sharp knife to trim the edges for a neat finish. Carefully slice the cake into two equal layers horizontally.

Spread a thin layer of frosting or jam over the first layer of the cake. Carefully place the second layer on top, then cut into small squares or rectangles to create individual petit fours.

In a mixing bowl, whisk together the powdered sugar, heavy cream, and vanilla extract until smooth. If too thick, add more cream to reach the desired consistency.

Place a wire rack over a baking sheet. Dip each petit four into the glaze, allowing excess to drip off. Let them sit for a few minutes to set slightly.

Roll out the pastel-colored fondant and cut it into small shapes to cover each petit four. Secure with a dab of frosting and sprinkle with colorful sprinkles.

Transfer finished petit fours to a serving platter or cake box, ensuring they are arranged neatly. Store at room temperature or in the fridge.