Reduce the cider: In a small saucepan, simmer apple cider until reduced by half, about 10–12 minutes. You should have about ½ cup. Let it cool slightly.
Make the dough: In a medium saucepan, combine reduced cider, butter, salt, and brown sugar. Bring to a simmer. Add flour all at once and stir vigorously until the dough pulls away from the sides and forms a smooth ball, about 2 minutes.
Cool & add eggs: Transfer dough to a bowl and let cool for 5 minutes. Beat in eggs one at a time until smooth and glossy, then stir in vanilla, cinnamon, and nutmeg.
Heat the oil: Pour oil into a heavy pot to about 2 inches deep. Heat to 350°F.
Fry the zeppole: Drop heaping tablespoons of dough into hot oil, working in batches. Fry 3–4 minutes, turning occasionally, until puffed and golden brown. Drain on paper towels.
Coat in spiced sugar: While still warm, toss zeppole in the cinnamon-sugar mixture until evenly coated.
Make the caramel: Drain soaked dates (reserve a few tablespoons of soaking water). In a small saucepan, melt butter over medium heat until golden and nutty, 3–4 minutes. Remove from heat.
Blend: Add dates, browned butter, warm milk, vanilla, and salt to a blender or food processor. Blend until silky, adding a splash of soaking water if needed to loosen. Taste and adjust salt. Finish with flaky sea salt on top.
Serve: Pile the warm zeppole on a platter, dust with extra cinnamon sugar, and serve with the salted brown butter date caramel for dipping.