Preheat the oven to 350°F (180°C).
In a microwave-safe bowl, combine the butter and chocolate. Microwave in 30-second intervals, stirring between each, until fully melted. Let it cool.
Add the egg yolks, 125g of sugar, and vanilla extract to the chocolate mixture. Stir until well combined.
In a separate bowl, use a hand or stand mixer to whip the egg whites until foamy. Gradually add 20g of sugar and continue whipping until stiff peaks form.
Gently fold the whipped egg whites into the chocolate mixture in two additions, being careful not to deflate the batter.
Line a cake pan (8in) with parchment paper and pour in the batter and bake for 20-25 minutes, or until an inserted toothpick comes out with a few crumbs
Refrigerate the cake for 4hrs or overnight.
Dust with powdered sugar before serving.