Preheat oven to 350°F (175°C). Grease an 8 inch round cake pan and line the bottom with parchment.

Melt the chopped dark chocolate gently and let it cool slightly.

In a mixing bowl, beat the softened butter with 1/2 cup sugar until lighter in color. Beat in the egg yolks one at a time, then mix in the vanilla extract and melted chocolate.

Stir in the flour and a pinch of salt until just combined.

In another bowl, beat the egg whites until foamy, then slowly add 1/4 cup sugar and beat to soft peaks.

Gently fold the egg whites into the chocolate batter in a few additions.

Pour the batter into the prepared pan and bake for about 30 to 40 minutes. The center should spring back lightly. Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.

Warm the apricot jam until smooth, then slice the cooled cake horizontally. Brush jam on the bottom layer, place the top back on, and brush jam over the top and sides. Let it sit for 15 minutes to set.

For the glaze, bring sugar and water to a simmer for about 5 minutes, then remove from heat and stir in the chocolate until smooth. Let it cool for a couple of minutes to thicken slightly.

Place the cake on a rack and pour the glaze over the top, guiding it gently over the sides. Let it set before slicing.