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Preheat your oven to 350°F. Lightly grease a 9” square pan.
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To make the cake: In a medium-sized bowl, whisk together the flour, sugar, cocoa powders, salt, and baking soda.
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In a separate bowl, whisk together the vanilla, vinegar, olive oil, water, and eggs. Pour the wet ingredients into the bowl of dry ingredients, stirring until thoroughly combined. Pour the batter into the prepared pan.
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Bake the cake for 24 to 28 minutes, until a toothpick or paring knife inserted into the center comes out clean, or with a few moist crumbs clinging to it.
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To make the topping: While the cake is baking, pulse the chocolate in a food processor until finely ground. Stir in the granulated sugar and mix to combine. If you don’t have a food processor (or blender), you can simply chop the chocolate by hand; try to chop it as finely as possible before mixing in the granulated sugar.
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Immediately after the cake is done, sprinkle the ground chocolate mixture evenly over the top of the hot cake. Sprinkle sparkling sugar on top of the chocolate mixture, if desired.
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Return the cake to the oven for about a minute, just until the topping starts to settle in and melt slightly.
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Remove the cake from the oven and allow it to cool to the touch before placing it in the refrigerator just until the chocolate topping is set, about 10 to 15 minutes.
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Storage information: Store the cake, well covered, at room temperature for several days; freeze for longer storage.
