Make the cookie crumble by mixing together the almond flour, cocoa powder, melted butter, powdered erythritol, and pinch of salt in a small bowl until it forms dark, clumpy crumbs. Break apart any large clumps so you have pieces roughly the size of small cookie chunks.
In a microwave-safe mug (at least 12 oz), whisk together the almond flour, coconut flour, granulated erythritol, baking powder, and salt for the vanilla cake base.
Add the egg, melted butter, heavy cream, and vanilla extract. Stir until the batter is smooth. Reserve about one third of the cookie crumble for topping, then fold the rest into the batter so the dark cookie pieces are scattered throughout the white vanilla cake.
Microwave on high for 60 to 70 seconds, until the cake is set and puffed but still tender in the center.
Top the warm cake with the sweetened whipped cream and scatter the reserved cookie crumble pieces on top. The contrast of the cool cream, crunchy dark cookie bits, and warm vanilla cake is what makes this one unforgettable.
