Choose a premium chocolate bar for this simple one-layer cake. Make up to one day ahead, and store in an airtight container.
1. Preheat oven to 350°.
2. Coat 1 (9-inch) round cake pan with cooking spray; dust with 2 teaspoons flour.
3. Weigh or lightly spoon 5.3 ounces flour into dry measuring cups; level with a knife. Combine flour and baking powder in a bowl; stir with a whisk. Place 1/2 cup sugar and 1/4 cup butter in a large bowl; beat with a mixer at medium speed until well blended (about 5 minutes). Add egg; mix well. Add flour mixture to sugar mixture alternately with milk, beginning and ending with flour mixture. Stir in vanilla.
4. Beat egg whites with a mixer at high speed until foamy using clean, dry beaters. Gradually add 3 tablespoons sugar, beating until stiff peaks form. Gently fold half of egg white mixture into flour mixture; fold in remaining egg white mixture. Grate 1/2 ounce chocolate; fold grated chocolate into batter. Pour batter into prepared pan. Bake at 350° for 23 minutes or until a wooden pick inserted near center comes out clean. Cool in pan 10 minutes. Remove from pan; cool completely on a wire rack.
5. Chop the remaining 2 ounces chocolate. Combine chopped chocolate and 1 tablespoon butter in a microwave-safe bowl. Microwave at MEDIUM 1 minute or until chocolate melts, stirring every 15 seconds. Spread chocolate mixture over top of cake; sprinkle evenly with sea salt. Cut into 8 wedges.
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