To prepare, preheat the oven to 180C. Next, brush a 23cm springform pan with melted butter and line its base with non-stick paper.
Gently stir chocolate and unsalted butter in a heatproof bowl over simmering water until it's smooth. Then, allow it to cool.
Using an electric beater, whisk egg yolks and brown sugar in a large bowl for 3 minutes until the mixture thickens and becomes creamy. Incorporate vanilla, and gently fold in the chocolate mixture, almond meal, and a tablespoon of bourbon. In a separate clean, dry bowl, whisk the egg whites until soft peaks form. Gradually fold one-third of the whipped egg whites into the chocolate mixture, then gently fold in the remaining egg whites.
Evenly spread the mixture in the pan, then bake for 45-55 minutes or until a skewer inserted into the center emerges nearly clean. Allow it to cool in the pan.
In the meantime, heat caster sugar in a saucepan over medium heat, swirling it for 5-10 minutes until it turns golden and the sugar dissolves. Allow it to cool slightly.
In a saucepan, merge the butter, cream, and the remaining bourbon. Gently cook for 5-10 minutes, ensuring a smooth blend. Afterward, take it off the heat, introduce extra chocolate, and, while still on low heat, whisk until it achieves a seamless consistency. Allow it to cool for a short while.
Slice the cake into wedges and serve it alongside the sauce for a delightful treat.