Preheat the oven to 350 degrees F. Grease a 6-cup jumbo muffin tin with nonstick cooking spray.
Microwave the cashew butter in 30-second increments until it is slightly warm and easy to stir.
Beat together the cashew butter and eggs with an electric mixer fitted with the paddle attachment until smooth and fluffy, about 5 minutes. Beat in the maple syrup, cocoa powder, vanilla, baking soda and salt until the mixture is smooth, thick and glossy (like a brownie batter).
Pour the batter into the prepared muffin tin and bake until the muffins puff up, the tops are cracked and a toothpick inserted in the center comes out clean, 15 to 20 minutes. Let the muffins cool in the pan for 5 minutes, then transfer them to a cooling rack.
You can substitute almond, pecan or another nut butter, but make sure it's a very smoothly ground butter--nothing coarse or crunchy.