Preheat the oven to 350F (177C) and line a cookie sheet with parchment paper.
Combine all ingredients for birthday cake crumbs in a medium bowl and mix with a hand mixer until small clusters form.
2/3 cup (80g) all-purpose flour, 1/4 cup (50g) granulated sugar, 1/4 tsp baking powder, 1/4 cup (57g) butter, 2 tsp vanilla, 1 tsp rainbow sprinkles
Spread the crumbs on the cookie sheet and bake for about 10 minutes. Remove them from the oven, use a fork to break up the large chunks (scramble them up a little) and return them to the oven for about 3-5 minutes until golden brown. Remove them and allow them to cool completely- they will crisp up as they cool- then break them into crumbs.
Place 1/2 cup of water in a saucepan over medium heat. Place the white chocolate with the sweetened condensed milk, butter and salt in a glass bowl on top of the saucepan, creating a double boiler. Stir the mixture frequently so the chocolate doesn't burn.
18 oz white chocolate, 14 oz (1 can) sweetened condensed milk, 1/2 cup (115g) butter, 1/8 tsp salt
Meanwhile line an 8x8" pan.
Once the mixture is completely smooth, fold in HALF of the birthday cake crumbs and spread the mixture evenly in the prepared pan. Top the fudge with the rest of the crumbs and a few more sprinkles.
1 batch of birthday cake crumbs, 1 Tbsp rainbow sprinkles
Refrigerate the fudge for a few hours (or overnight) before slicing. Serve cold or at room temperature.