For the Birthday Cake Sand:
Preheat the oven to 300°F.
Combine the sugars, flour, baking powder, salt and sprinkles in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until well-combined.
Add the oil and vanilla, and mix to combine. The wet ingredients will act as glue to help the dry ingredients form small sandy clusters; continue paddling until the small clusters form.
Spread on a parchment paper-lined baking sheet, then bake for 15 minutes. Let cool completely, then use a food processor to grind into a sand-like texture. (The sand will not adhere to the white chocolate shell in the birthday cake truffle recipe unless finely ground.) Store in an airtight container at room temperature for up to 1 week, or refrigerated or frozen for up to 1 month.
For the Truffles:
Combine the milk and vanilla in a medium bowl; set aside about 2 tablespoons vanilla milk. Add the cake scraps to the bowl with the other 2 tablespoons vanilla milk and, using your hands, toss until moist enough to knead into a ball, gradually adding the reserved 2 tablespoons vanilla milk if needed. (Depending on the moistness of the cake scraps, more or less of the liquid binder may be needed.)
Using a small ice cream scoop, portion out 12 even balls, each half the size of a ping-pong ball. If using the sprinkles, add them to the center of each ball and seal the edges to enclose. Roll each ball between your palms to shape and smooth into a round sphere.
Put the Birthday Cake Sand in a bowl.
Wearing latex gloves, put 2 tablespoons melted white chocolate in the palm of your hand and roll each ball between your palms, coating it in a thin layer of melted chocolate; add more chocolate as needed.
Put 3 or 4 chocolate-covered balls at a time into the bowl of ground crumbs. Immediately toss to coat, before the chocolate shell sets and no longer acts as a glue. (If the chocolate sets before coating, just coat the ball in another thin layer of melted chocolate.)
Refrigerate for at least 5 minutes to fully set the chocolate shells. Serve immediately or refrigerate in an airtight container for up to 1 week.
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