Prep the oven and baking sheets. Preheat your oven to 350°F (175°C). Then, arrange two oven racks in the upper and lower thirds of the oven. Line two baking sheets with parchment paper.
Prepare the batter. In a large mixing bowl, beat the butter, sugar, egg, and vanilla extract using an electric mixer on medium speed for 3 to 5 minutes until the mixture is fluffy and lighter in color. Mix the flour, salt, and baking soda in a different bowl. Gradually add the dry ingredients to the butter mixture, alternating with the milk, beginning and ending with the dry ingredients. Finally, fold in the strawberry gelatin powder until fully combined. The batter will be thick, somewhere between a cake batter and cookie dough.
Scoop and bake. Using a ¼ cup measuring scoop, portion the batter onto the prepared baking sheets, spacing each scoop three inches apart to allow for spreading. You should have 12 scoops in total. Bake for about 18 minutes, rotating the baking sheets halfway through. The cakes should be set, with edges slightly golden brown, and a cake tester should come out clean when inserted into the center.
Cool the cakes. Let the cakes cool on the baking sheets for 1-2 minutes to firm up slightly, making them easier to handle. Then, transfer them to wire racks to cool completely.