01 - Preheat oven to 350°F (175°C). Lightly grease an 8-inch square baking pan and line with parchment paper.
02 - In a large mixing bowl, whisk together flour, granulated sugar, baking powder, and salt. In a separate bowl, whisk together milk, melted butter, egg, and vanilla extract. Gradually incorporate the wet ingredients into the dry ingredients until just combined. Gently fold in the rainbow sprinkles.
03 - Pour the batter evenly into the prepared baking pan. Bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely.
04 - While the cake cools, prepare the frosting. In a bowl, beat the softened butter until creamy. Gradually add the powdered sugar, milk, vanilla extract, and salt, beating until the frosting is smooth and fluffy.
05 - Crumble the cooled cake into a large bowl. Add approximately two-thirds of the prepared frosting to the cake crumbs and mix until the mixture holds together when pressed, avoiding excessive moisture.
06 - Lay out the flour tortillas. Spoon a generous portion of the cake-frosting mixture down the center of each tortilla. Sprinkle with additional rainbow sprinkles and the crushed crispy rice cereal or wafer cookies, if using.
07 - Drizzle or spread a small amount of the remaining frosting over the filling in each tortilla. Fold the sides of the tortilla inward, then roll tightly from the bottom to create a wrap.
08 - Slice the wraps in half and serve immediately. Alternatively, chill for 15 minutes to allow the wraps to set before serving.