Prepare the Ganache Core: In the small saucepan, combine 1.5 oz (45 g) of the chopped dark chocolate, the heavy cream, 2 tablespoons of butter, and 2 tablespoons of the powdered sweetener. Melt over very low heat, stirring constantly until the mixture is completely smooth and glossy.
Freeze the Molten Centers: Carefully pour the liquid ganache mixture evenly into the 5 silicone muffin liners (about 1.5 tablespoons each). Place the liners on a small tray and transfer to the freezer. Freeze for 20–25 minutes, or until the ganache discs are solid.
Whip the Mousse Base: While the cores freeze, combine the softened cream cheese and the remaining 4 tablespoons of butter in a mixing bowl. Using the electric hand mixer, beat on medium-high speed for about 2 minutes until the mixture is airy, pale, and free of lumps.
Combine Dry Components: In a separate bowl, whisk together the remaining powdered sweetener, the unsweetened cocoa powder, almond flour, espresso powder, and sea salt until thoroughly blended.
Finish the Chocolate Casing: Add the dry ingredient blend to the whipped cream cheese mixture, mixing on low speed until just incorporated. Melt the remaining 2 oz (55 g) of dark chocolate in the now-empty saucepan over low heat until just melted. Pour the melted chocolate into the bowl and fold it through with a spatula until the color is uniform and rich.
Assemble the Cups: Remove the firm ganache discs from the silicone liners and set them aside. Press approximately two-thirds of the chocolate mousse mixture into the bottom and up the sides of the same liners, forming a hollow nest. Place one frozen ganache disc into the center of each nest.
Seal and Chill: Distribute the remaining mousse evenly, topping each cup to completely enclose the ganache core. Use a small spatula or the back of a spoon to create a smooth, flat surface.
Final Set for Serving: Refrigerate the assembled cups for a minimum of 2 hours, allowing them to become fully firm. To serve, gently peel back the silicone liner. For the ideal molten effect, let each cup rest at room temperature for 5-10 minutes before plating. Dust with cocoa powder just before serving.