Step 1: Prepare the Cake Batter

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and line a 9-inch (23 cm) springform pan with parchment paper.
2. Sift Dry Ingredients: In a medium bowl, sift together the flour, cocoa powder, and baking powder. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, beat the softened butter and powdered sugar until light and fluffy. Add the egg yolks, one at a time, beating well after each addition. Stir in the vanilla extract.
4. Whip Egg Whites: In a separate bowl, whip the egg whites and salt until soft peaks form. Gradually add the granulated sugar and continue beating until stiff peaks form.
5. Combine Ingredients: Fold the dry ingredients into the butter mixture in three additions, alternating with the whipped egg whites. Gently fold until the batter is smooth and fully combined, taking care not to deflate the egg whites.

Step 2: Bake the Cake

1. Pour the batter into the prepared springform pan and smooth the top with a spatula.
2. Bake for 40-50 minutes, or until a toothpick inserted into the center comes out clean.
3. Let the cake cool in the pan for 10 minutes, then remove it from the pan and transfer it to a wire rack to cool completely.

Step 3: Prepare the Apricot Filling

1. In a small saucepan, warm the apricot jam with 2 tablespoons of water over low heat. Stir until smooth and spreadable. Strain the mixture through a fine-mesh sieve to remove any lumps or fruit pieces for a smoother finish.
2. Once the cake is completely cool, slice it horizontally into two even layers using a serrated knife.
3. Spread a generous layer of the apricot jam on the bottom layer, then place the top layer back on. Brush a thin layer of the remaining apricot jam over the top and sides of the cake to create a smooth base for the glaze.

Step 4: Make the Chocolate Glaze

1. Heat the heavy cream in a small saucepan over medium heat until it begins to simmer (do not boil).
2. Remove from heat and add the chopped dark chocolate. Let sit for 2 minutes, then stir until smooth and glossy. Stir in the butter for added shine.
3. Place the cake on a wire rack set over a baking sheet. Pour the warm glaze over the cake, letting it drip down the sides. Use a spatula to ensure the entire cake is evenly coated.

Step 5: Let the Glaze Set

1. Allow the glaze to set at room temperature for at least 1 hour, or refrigerate for 30 minutes if you’re in a hurry.
2. Transfer the cake to a serving platter and slice with a sharp knife, wiping the blade clean between cuts for neat slices.