Prepare the Cake:

1. Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving overhang on the sides for easy removal. Lightly grease the parchment.
2. In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
3. In a large mixing bowl using an electric mixer, cream the softened butter and granulated sugar together on medium-high speed for 3-4 minutes until light and fluffy. The mixture should be pale and increased in volume.
4. Add eggs one at a time, beating well after each addition. Scrape down the sides of the bowl as needed. Mix in the vanilla extract.
5. With the mixer on low speed, add the flour mixture in three additions, alternating with the milk in two additions, beginning and ending with the flour. Mix just until combined—don’t overmix.
6. Pour the batter into the prepared pan and spread evenly with a spatula. Tap the pan gently on the counter to remove air bubbles.
7. Bake for 23-27 minutes, until the cake is golden and a toothpick inserted in the center comes out clean. The cake should spring back when lightly touched.
8. Cool the cake in the pan for 10 minutes, then use the parchment overhang to lift it onto a wire rack to cool completely.

Prepare the Filling:

9. In a mixing bowl, beat the softened butter on medium speed until creamy, about 2 minutes.
10. Gradually add the sifted powdered sugar, beating on low speed to prevent a sugar cloud, then increase to medium speed.
11. Add heavy cream and vanilla extract, beating until the buttercream is light and fluffy, about 3 minutes. Set aside.

Assemble the Layers:

12. Once the cake is completely cool, use a sharp serrated knife to trim all four edges to create straight sides. Cut the cake horizontally into three equal rectangles (approximately 4×12 inches each).
13. Place one cake layer on a clean work surface. Spread half of the raspberry jam evenly over the top, then spread half of the buttercream over the jam.
14. Place the second cake layer on top and repeat with the remaining jam and buttercream.
15. Top with the third cake layer and press down gently. Wrap the assembled cake tightly in plastic wrap and refrigerate for at least 1 hour or overnight to firm up.

Cut and Prepare for Glazing:

16. Remove the chilled cake from the refrigerator. Using a sharp knife dipped in hot water and dried (for clean cuts), trim the edges again if needed, then cut the cake into 1½-inch squares. You should get approximately 48 petit fours.
17. Place the cake squares on a wire rack set over a large baking sheet (to catch glaze drips). Space them about 1 inch apart.

Prepare and Apply the Glaze:

18. In a medium saucepan over low heat, combine the powdered sugar, corn syrup, milk, and vanilla. Stir constantly until smooth and the mixture reaches 92-98°F on a kitchen thermometer. The glaze should be thin enough to pour but thick enough to coat the cakes.
19. If desired, divide the glaze into separate bowls and tint with gel food coloring. Add more milk a teaspoon at a time if the glaze becomes too thick.
20. Working quickly, pour the warm glaze over each petit four, covering the tops and sides completely. You can use a fork or dipping tool to turn the cakes if needed. Let excess glaze drip off.
21. Allow the glaze to set for about 30 minutes at room temperature. You can carefully lift each petit four with a small offset spatula and transfer to a serving platter.
22. Once the first coat is set, apply a second thin coat of glaze for complete coverage if desired. Let set completely.
23. Decorate with royal icing, edible flowers, gold leaf, or other embellishments if desired.