Preheat oven to 325°F (170°C). Line bottoms of 2 (8-inch) round cake pans with parchment paper.

For cake layers: In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites at medium-high speed until foamy. Reduce mixer speed to medium-low, and add ⅓ cup (67 grams) sugar in a slow, steady stream. Increase mixer speed to medium-high, and beat until medium peaks form.

Meanwhile, in the top of a double boiler, heat chocolate and butter over simmering water, stirring occasionally, until fully melted. Remove from heat. Whisk in remaining ½ cup (100 grams)sugar until well combined. Let cool slightly, about 5 minutes. Whisk in egg yolks, sour cream, and vanilla until well combined.

In a medium bowl, sift together flour, baking powder, and salt. Gradually whisk flour mixture into chocolate mixture. (Mixture will be thick.) Fold in one-third of egg white mixture. (This will thin out the batter.) Carefully fold in remaining egg white mixture in two additions. Divide batter between prepared pans (about 394 grams each). Run a small offset spatula through batter to help release any large air bubbles, and spread smooth.

Bake until fragrant and top is firm to the touch when gently pressed, about 20 minutes. Let cool in pans for 10 minutes. Run a small offset spatula or thin knife around sides of pans to loosen layers. Let cool completely in pans on wire racks.

For syrup: In small saucepan, bring sugar and ¼ cup (60 grams) water to a boil over medium-high heat, whisking occasionally until sugar dissolves. Remove from heat; let cool completely.

Strain preserves through a fine-mesh sieve into a small bowl, discarding solids. (You should have about ¾ cup or about 240 grams after straining.)

Invert cooled cake layers onto a wire rack set on a rimmed baking sheet. Using a pastry brush, brush cooled syrup onto each layer. Spread ½ cup (160 grams) strained preserves onto 1 cake layer. Top with remaining cake layer, syrup side up. Using an offset spatula, spread remaining ¼ cup (80 grams) strained preserves on top and sides of cake. Refrigerate until set, about 30 minutes.

For glaze: In the top of a double boiler, heat chocolate, butter, and corn syrup over simmering water, stirring occasionally, until chocolate and butter are melted and mixture is smooth. Remove from heat; let cool for 5 minutes. Spoon about ¼ cup (about 80 grams) glaze into a pastry bag fitted with a small round piping tip (Wilton No. 4).

Pour remaining warm glaze onto cake, letting glaze run over sides and using an offset spatula to fully cover cake. Let stand at room temperature for 5 minutes.

Using a cake lifter, carefully transfer cake to desired serving plate. Garnish edges of cake with chocolate sprinkles, if desired. Pipe reserved glaze onto cake as desired. Refrigerate until glaze is set, 10 to 15 minutes. Let stand at room temperature for 20 to 25 minutes before serving. Refrigerate in an airtight container for up to 3 days.