Position an oven rack in the bottom third of your oven and preheat the oven to 325°F. Have a 9 or 10-inch ungreased tube pan (16-cup capacity) ready.

In a food processor or blender, pulse the sugar until it becomes superfine. Remove 200g of the sugar and sift it into a bowl through a fine mesh strainer, discarding any large granules. Set aside.

Add the cake flour and salt to the food processor with the remaining sugar and pulse about 10 times to aerate the dry ingredients. Sift this mixture into a separate bowl with a fine mesh strainer and discard any large pieces. Set aside.

In the bowl of a stand mixer fitted with the whisk attachment, add the room temperature egg whites and cream of tartar. Mix on low speed until foamy, about 1 minute, then increase to medium-high. Gradually add the reserved superfine sugar, beating until the egg whites reach soft peaks about 5-6 minutes.

To the egg whites, add the flour mixture in 3 additions, gently folding it into the egg whites with a rubber spatula. Take care not to deflate the batter.

Carefully pour the runny, room temperature sourdough discard and vanilla extract on top of the batter and gently fold it until just incorporated.

Pour the batter into the ungreased tube pan and gently smooth the top with the rubber spatula.

Bake for 40–45 minutes or until the top is lightly golden and a toothpick comes out clean. Turn the cake halfway through the bake.

Remove from the oven and immediately turn the pan upside down. Allow the cake to cool upside down for at least 3 hours.

Once fully cooled, run a knife around the edges to release the cake. If desired, dust the top of the cake with powdered sugar. Slice with a serrated knife and serve with whipped cream and fresh berries.