Melt the butter and dark chocolate together in a medium saucepan over low heat, stirring occasionally. As soon as everything is melted and smooth, remove from heat. Let it sit for five minutes to cool.
In a small mixing bowl, whisk together eggs, sugar and vanilla. Slowly stir the mixture in to the cooled chocolate and butter. Beat with a wooden spoon until combined.
Fold in the flour and salt until combined, then fold in the white chocolate chips.
Pour the batter into your waiting pan and bake for about 25 minutes. You should have a flaky crust on the top. Don't overbake them. The gooeyness is what makes them so dreamy.
Do you very best to allow them to cool completely. I refrigerated mine for a few hours before cutting and storing in a tin. If you want to be fancy, sprinkle some powdered sugar over the top.