Cream the butter and sugar: In a large mixing bowl, beat the softened butter, confectioners' sugar, and vanilla extract until light, smooth, and creamy.

Add the water: With the mixer running on low speed, slowly add the water until fully incorporated.

Mix in the dry ingredients: Gradually add the flour and cornstarch, mixing until a soft dough comes together. Avoid overmixing.

Chill the dough: Gently shape the dough into a loaf, similar to a sliceable loaf of bread. Wrap tightly in plastic wrap and refrigerate for at least 1 hour.

Preheat the oven: Heat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Cut into fairy bread shapes: Remove the chilled dough from the refrigerator and slice it into even rectangular "bread slices." Cut each rectangle diagonally in half to create the classic fairy bread triangles.

Bake: Arrange the cookies on the prepared baking sheet and bake for 20 minutes, rotating the pan halfway through. If the centers need a little more time, reduce the oven temperature slightly and bake for an additional 3 to 5 minutes, watching carefully so the edges don't brown too much.

Cool completely: Transfer the cookies to a wire rack and let them cool completely before decorating with frosting and sprinkles.

Make the outline icing: In a medium bowl, whisk together the powdered sugar, 2 tablespoons of milk, corn syrup, and vanilla extract until smooth. The icing should be thick enough to hold its shape. If it's too thin, add a little more powdered sugar until you reach the right consistency.

Prepare the piping bag: Transfer about one-third of the icing to a sandwich bag or piping bag and snip a very small hole in one corner.

Outline the cookies: Pipe a border around the edge of each cooled cookie. Let the outline set for 5 to 10 minutes to create a barrier for the flood icing.

Thin the remaining icing: Add the remaining tablespoon of milk to the icing left in the bowl and whisk until it reaches a slightly thinner, pourable consistency.

Flood the cookies: Transfer the thinner icing to another sandwich bag or piping bag, snip a small hole, and fill the center of each outlined cookie with icing. Use a toothpick to gently spread it into the corners if needed.

Add the sprinkles: Let the icing sit for a minute or two, then generously cover each cookie with rainbow sprinkles so no icing is visible.

Let the icing set: Allow the cookies to sit until the icing is completely firm before serving or storing. Enjoy!