Prep the pan: Heat oven to 350°F (175°C).
Line an 8x8-inch pan with parchment, leaving overhang for easy lifting.
Melt butter and chocolate: In a microwave-safe bowl, melt butter with half the chocolate in 20–30 second bursts, stirring until smooth. Let cool 2–3 minutes.
Whisk in sweetener: Add erythritol or allulose and whisk well. The mixture will look grainy if using erythritol—that’s okay.
Add eggs and vanilla: Whisk in eggs one at a time, then vanilla.
Mix until glossy and slightly thick.
Dry ingredients: Stir in almond flour, cocoa powder, baking powder, and salt until just combined. Do not overmix.
Fold in chips: Fold in remaining chocolate chips and any nuts if using.
Bake: Spread batter evenly in the pan. Bake 18–24 minutes, until the top is set and a toothpick comes out with a few moist crumbs.
Cool completely: Let the brownies cool in the pan for at least 30–45 minutes.
Lift out with parchment and slice.
Finish (optional): Sprinkle with flaky sea salt or dust with extra cocoa powder.