Generously grease a 9.5 x 7.5 inch (24 x 19 cm) baking pan.
Evenly spread marshmallows on the bottom of the pan.
Mix the thawed berries with the dry gelatin and set aside.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt.
In another bowl, combine sugar, softened butter, eggs, vanilla extract, and milk, and blend at medium speed until well combined.
Add the dry ingredient mixture to the wet ingredients and beat for 2 minutes until well combined.
Pour the cake batter evenly over the marshmallows.
Spoon the strawberry mixture evenly over the batter.
Bake in a preheated oven at 175°C (347°F) or 155°C (311°F) fan-assisted, for 40-45 minutes until golden brown and toothpick inserted in center comes out clean.
Serve warm or cool with ice cream, or whipped cream, or more fresh strawberries for added flavor.
Notes
Grease the pan well: Don’t forget the corners and sides to keep the marshmallows from sticking.Use room-temperature ingredients: Butter, eggs, and milk mix better when they’re not cold, giving your cake a smoother texture.Layer carefully: Gently pour the batter over the marshmallows, and use a spoon to spread the strawberry gelatin evenly for a balanced flavor.Cool a bit before serving (if eating warm): Let the cake sit for about 10 minutes so the marshmallows can settle, making it easier to slice.Store leftovers properly: Keep them in an airtight container—at room temp for 2 days or in the fridge for up to 5. Warm slices for a few seconds in the microwave to enjoy them again!