Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch pan with parchment, leaving overhang for easy lifting.
Melt butter: In a medium bowl, microwave butter until melted and warm, not sizzling.
Let cool 1–2 minutes.
Sweeten: Whisk in the erythritol until mostly dissolved. A few grains are fine.
Add eggs and vanilla: Whisk in eggs one at a time, then add vanilla. Mix until glossy and smooth.
Dry ingredients: Add almond flour, cocoa, salt, and baking powder.
Stir with a spatula until just combined and thick.
Optional mix-ins: Fold in sugar-free chocolate chips or nuts for extra texture.
Bake: Spread batter evenly in the pan. Bake 18–22 minutes, until the center is just set and a toothpick comes out with a few moist crumbs.
Cool completely: Let brownies cool in the pan at least 30–45 minutes. Lift out and slice with a sharp knife, wiping between cuts.