Preheat the oven to 350°F (180°C). Grease an 8-inch (20 cm) cake tin and line with parchment paper, leaving about 2 inches (5 cm) of overhang to help remove the cake after baking.

Place the chopped unsweetened chocolate, butter, and coconut milk in a heatproof bowl. Heat gently until melted and smooth—either in the microwave in 20–30 second bursts, stirring between bursts, or in a small saucepan over low heat—then remove from heat.

Stir the allulose and the 1/4 teaspoon salt into the warm chocolate mixture until mostly dissolved. Add the vanilla extract and espresso powder if using, and stir to combine. Let the mixture cool slightly (warm but not hot).

Add the eggs one at a time, beating or whisking gently after each addition until the egg is fully incorporated and the batter is smooth.

Add the cocoa powder and fold it in until just combined. Do not overmix.

Fold 1 cup of the chocolate chips into the batter; reserve 1/4 cup chocolate chips to sprinkle on top.

Pour the batter into the prepared tin and smooth the top. Scatter the reserved 1/4 cup chocolate chips evenly over the surface.

Bake for 25 minutes, until a gentle crust has formed on top and the center is mostly set (the center may still be slightly soft).

Remove the cake from the oven and allow it to cool completely in the tin on a wire rack. Use the parchment overhang to lift the cake from the tin, transfer to a plate, slice, and serve.