Preheat your oven to 320°F (160°C). Line a 9-inch (or 10-inch) round pan with parchment paper and grease the sides.
In the meantime melt the chocolate: transfer the nut butter, the chopped chocolate, the cocoa powder and the espresso granules (if using) to a medium sized pot and heat gently over low heat, stirring very often until the chocolate is melted. Remove from heat.
Combine the two mixtures: Add half of the fluffy egg mixture to the pot with the melted chocolate and mix until well combined. The mixture will thicken, but that’s normal. Add the rest of the egg mixture and fold with a spatula using circular motions until homogenized.
Bake: Pour the mixture to the baking pan and bake for 40-45 minutes. Let it rest for 10 minutes, run a knife through the edges of the pan, invert on a rack and re-invert on a serving plate. Dust with icing sugar or cocoa and serve with a scoop of vanilla ice cream or a dollop of sweetened whipped cream with a touch of vanilla.
This cake is not too sweet, so it’s best served slightly warm and with a scoop of vanilla ice cream or a dollop of sweetened whipped cream. You can also serve it with this golden milk no-churn ice cream for a more interesting combination.
If you want more of the nut butter flavor and less of the chocolate, you can reduce the amount of cocoa to 1/4 of a cup.
How to make homemade nut butter: Process in the food processor 2 cups of warm, roasted hazelnuts, almonds, or peanuts until creamy. You may have to stop and stir the mixture 1-2 times. Add a couple tablespoons of a neutral-tasting oil to make the nut butter smoother.