Direction

1. For syrup, pour cherry juice, Prosecco, and sugar into a saucepan and bring to a boil over medium-high heat. Reduce heat and cook at a lively simmer until syrupy and reduced by at least half, about 40 minutes. Set aside 1 cup cherries for cake; add remaining cherries to syrup; allow to cool. Syrup can be made several days in advance.
2. Preheat the oven to 350 degrees F (180 degrees C). Line a 9x9-inch square baking pan with parchment.
3. Stir sugar and egg together in a bowl. Stir in the reserved 1 cup cherries. Add flour, baking soda, salt, vanilla, and butter and stir until flour disappears. Spread into the prepared baking pan.
4. Bake in the preheated oven until a toothpick inserted near the center comes out clean, 30 to 40 minutes. The cake will collapse in the center; this is expected. Cool on a wire rack to room temperature.
5. Cut cake into squares, and spoon a generous amount of syrup with cherries over each serving. Cake and syrup are best served warm or at room temperature.