Combine sugar and water in a small pan over medium heat. Bring to a simmer, stirring until sugar dissolves. Once simmering, remove from heat and cool syrup for 10 minutes.
Place cranberries in a large bowl and pour the syrup over the top. Stir gently to coat all the cranberries with the syrup. Let cool completely, cover with plastic wrap, and refrigerate for at least 2 hours or overnight.
Once cranberries have steeped, and are now cold, drain into a colander. Place sanding sugar in a shallow dish. Add the cranberries in small batches, rolling to coat with sugar. Spread sugared cranberries in a single layer on a parchment-lined baking sheet. Let cranberries sit at room temperature until dry, about 1 hour. Store cranberries in an airtight container in a cool, dry place for up to one week.
Use remaining icing to coat the ice cream cones. Sprinkle with silver sprinkles and place on cake. Top with frosted cranberries.