1. Preheat oven to 350°F and position rack in center. Line an 8×8-inch baking pan with parchment paper, leaving overhang on two sides. Spray lightly with cooking spray.

2. Bring eggs and butter to room temperature for 30-60 minutes before starting.

3. In a large mixing bowl, whisk together almond flour, cocoa powder, erythritol, baking powder, and salt until no lumps remain.

4. In a separate medium bowl, whisk eggs until slightly frothy, then add melted butter, almond milk, and vanilla extract, whisking until fully combined.

5. Pour wet ingredients into dry ingredients and stir with a spatula until just combined—don’t overmix.

6. Add boiling water last and stir gently until smooth. The batter will be thin, which is correct.

7. Pour batter into prepared pan and spread evenly. Tap pan gently on counter 2-3 times to release air bubbles.

8. Bake for 18-22 minutes until a toothpick inserted in center comes out with just a few moist crumbs.

9. Remove from oven and let cool in pan for 10 minutes on a wire rack.

10. Use parchment overhang to lift cake from pan onto cooling rack. Let cool completely, about 30-45 minutes.

11. While cake cools, prepare frosting by beating softened butter with cocoa powder until smooth.

12. Add heavy cream, powdered erythritol, vanilla, and salt. Beat on low speed initially, then increase to high speed for 2-3 minutes until light and fluffy.

13. Spread frosting evenly over cooled cake using an offset spatula or butter knife.

14. Top with chopped nuts if desired and let frosting set for 15 minutes.

15. Cut into 12 squares and serve. Store covered at room temperature for up to 3 days or refrigerated for 5-7 days.