Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line the bottoms with parchment paper and grease the sides.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, salt, and the dry strawberry gelatin powder until well combined.
Step 3: In a separate medium bowl, cream together the softened butter and vegetable oil until light and fluffy. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
Step 4: Gradually add the dry ingredients mixture to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. Mix on low speed until just combined. Be careful not to overmix.
Step 5: Gently fold in the finely chopped fresh strawberries (or strawberry puree) into the cake batter.
Step 6: Divide the batter evenly between the two prepared cake pans, smoothing the tops with a spatula.
Step 7: Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cakes comes out clean.
Step 8: Let the cakes cool in their pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure they are fully cooled before frosting.
Step 9: While the cakes cool, prepare the cream cheese frosting: In a large bowl, beat the softened cream cheese and softened butter together until smooth and creamy.
Step 10: Gradually add the sifted powdered sugar, one cup at a time, beating until smooth after each addition. Beat in the vanilla extract and pinch of salt.
Step 11: Add milk or heavy cream, 1 tablespoon at a time, until the frosting reaches your desired spreadable consistency. Beat until light and fluffy.
Step 12: Once the cake layers are completely cool, place one layer on your serving plate or cake stand. Frost the top evenly. Place the second cake layer on top and then frost the top and sides of the entire cake. Garnish with fresh strawberry slices, if desired.
Step 13: Serve immediately or store covered. Enjoy your delicious Strawberry Jell-O Cake!