Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan, or line with parchment paper.
Step 2: In a medium bowl, whisk together the sifted cake flour, baking powder, and salt. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy (about 3-5 minutes).
Step 4: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.
Step 6: Pour the batter evenly into the prepared baking pan. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Step 7: Let the cake cool in the pan for 10 minutes, then invert onto a wire rack to cool completely. This is crucial before cutting or frosting to prevent crumbling.
Step 8: For the cream filling, in a medium bowl, beat the softened butter until creamy. Gradually add the sifted powdered sugar, beating until well combined and fluffy. Mix in the vanilla extract and heavy cream until the filling is smooth and spreadable. Adjust cream consistency as needed.
Step 9: For the green vanilla frosting, in a large bowl, beat the softened butter until creamy and smooth. Gradually add the sifted powdered sugar, one cup at a time, mixing until combined after each addition. Add the vanilla extract and 3 tablespoons of heavy cream. Beat on medium-high speed until light and fluffy. Add green gel food coloring, a tiny bit at a time, mixing until you achieve your desired festive green shade.
Step 10: Once the cake is completely cool, use a Christmas tree-shaped cookie cutter (about 3-4 inches tall) to cut out individual cake trees. Carefully slice each cake tree horizontally through the middle using a serrated knife.
Step 11: Spread a generous layer of the cream filling on the bottom half of each cake tree. Place the top half back on, gently pressing down.
Step 12: Place the assembled cakes on a wire rack set over a baking sheet. Frost each cake with the green vanilla frosting. Use an offset spatula or a butter knife to spread evenly.
Step 13: Immediately sprinkle with red sprinkles while the frosting is still wet. Allow the frosting to set slightly before serving.