How to Make It
Make the cake: Preheat oven to 350°F. Grease a 9-inch round cake pan with butter and line with parchment paper. Whisk together flour, cocoa powder, baking soda, and kosher salt in a medium bowl. Whisk together granulated sugar, egg, and melted butter in a large bowl until pale; whisk in 1 cup wine and vanilla. Add flour mixture, and whisk to combine.
Scrape batter into prepared pan, and smooth top. Bake until a toothpick inserted in center comes out clean, 45 to 50 minutes. Let cake cool completely.
Make the ganache: Place chopped chocolate, diced butter, and corn syrup in a large microwave-safe bowl. Microwave at high in 20-second intervals until melted, stirring between intervals. Stir in powdered sugar and 2 tablespoons wine. Let stand at room temperature until slightly thickened.
Invert cake onto a plate, and invert again onto a platter. Spread ganache on top, letting it drip down sides. Sprinkle with flaky sea salt, and let stand until ganache is set, about 30 minutes. Cut into wedges and serve.