Prepare the Sponge Cake

1. Preheat Oven: Preheat your oven to 350°F (175°C). Line an 8-inch round cake pan with parchment paper and grease the sides.
2. Whisk Egg Yolks and Sugar: In a mixing bowl, whisk together the egg yolks and sugar until pale and fluffy, about 4-5 minutes.
3. Sift Dry Ingredients: In another bowl, sift together the flour, cocoa powder, baking powder, and salt.
4. Beat Egg Whites: In a clean bowl, beat the egg whites until soft peaks form. Gradually add sugar and continue to beat until stiff peaks form.
5. Fold in Dry Ingredients: Gently fold the dry ingredients into the egg yolk mixture, then fold in the whipped egg whites in three additions. Be careful not to deflate the batter.
6. Bake: Pour the batter into the prepared cake pan and bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean.
7. Cool: Let the cake cool completely on a wire rack before trimming it to fit your entremet mold.

Prepare the Chocolate Mousse

1. Melt the Chocolate: Place the dark chocolate in a heatproof bowl. Melt it over a double boiler or in the microwave, stirring until smooth. Set aside to cool slightly.
2. Whip the Heavy Cream: In a separate bowl, whip 1/2 cup heavy cream to soft peaks. Set aside.
3. Heat the Egg Yolks and Sugar: In a small saucepan, combine the egg yolks and sugar. Heat over low heat while whisking continuously until the mixture thickens slightly.
4. Combine with Chocolate: Gradually whisk the egg yolk mixture into the melted chocolate. Stir in vanilla extract.
5. Fold in Whipped Cream: Gently fold the whipped cream into the chocolate mixture, being careful to maintain the mousse’s airy texture.

Prepare the Chocolate Glaze

1. Melt the Glaze Ingredients: In a small saucepan, heat the heavy cream until it begins to simmer. Pour it over the chopped chocolate and stir until smooth. Stir in the corn syrup until the glaze is shiny and fluid.
2. Cool Slightly: Let the glaze cool to room temperature before using. It should be pourable but not too hot when drizzling over the mousse.

Assemble the Entremet

1. Prepare the Mold: Line the inside of an entremet mold with plastic wrap or acetate strips for easy removal.
2. Layer the Sponge Cake: Cut the cooled sponge cake to fit the bottom of the mold. Place it as the base layer.
3. Add the Mousse: Pour the chocolate mousse over the sponge cake layer, smoothing the top with a spatula.
4. Chill: Refrigerate the entremet for at least 3-4 hours to allow the mousse to set completely.
5. Glaze the Mousse: Once the mousse has set, pour the cooled chocolate glaze over the mousse, letting it drip down the sides.
6. Chill Again: Place the entremet back into the refrigerator for another hour or two until the glaze is firm.