Step 1

Make the creme anglaise: Bring milk, sugar, orange zest, and a pinch of salt to a simmer in a medium saucepan. Stir, and remove from heat. Cover, and let stand 1 hour.

Step 2

Reheat milk mixture and whisk into egg yolks in a bowl; return to pan. Cook over medium-high heat, stirring, until just before mixture begins to simmer, about 2 minutes. Reduce heat to medium; cook, stirring, until mixture is thick and coats the back of a wooden spoon, about 4 minutes. Pour mixture through a fine sieve into a bowl; refrigerate until chilled, about 2 hours or up to 3 days.

Step 3

Make the cake: Preheat oven to 400 degrees. Coat a 6-cup baking or gratin dish with cooking spray. In a heatproof bowl set over (not in) a saucepan filled with 2 inches simmering water, melt butter and chocolate with 1/2 teaspoon salt, whisking to combine.

Step 4

Beat eggs, egg yolks, and sugar with a mixer on medium speed until light, frothy, and thick, 6 to 8 minutes. Add chocolate mixture, beating to combine. Add flour, beating to combine.

Step 5

Transfer batter to dish; bake until set around edge but still soft in center, 10 to 12 minutes. Let cool for 3 minutes. Serve warm with creme anglaise.