Preheat the oven to 325ºF and grease a 9x13 glass or ceramic baking pan. (If using metal, the cake may bake through faster so watch it carefully).
In a large bowl, whisk together the almond flour, sweetener, cocoa powder, baking powder, and salt. Stir in the eggs, melted butter, water, and vanilla extract until well combined. Spread evenly in the prepared pan.
In another large bowl, beat the cream cheese, peanut butter and butter together until smooth. Beat in the powdered sweetener until well combined, then beat in the almond milk and vanilla extract.
Dollop this mixture over the cake mixture by the spoonful, then use a knife to swirl into the cake mixture.
Sprinkle the top with the chocolate chips and chopped peanuts. Bake 35 to 45 minutes, or until the cake edges are set. The center will still be soft and slightly jiggly.
Sprinkle with the chopped peanut butter cups while the cake is still warm. Let cool before serving.
Storage Information: Store this cake in the fridge, tightly covered, or up to a week. It can also be frozen for several months.