Prep your pan and oven. Preheat the oven to 350°F (175°C). Line an 8x8-inch pan with parchment, letting some hang over the sides for easy lifting.

Melt the butter and chocolate. In a heat-safe bowl, melt the butter and chopped chocolate together in short microwave bursts (or over a double boiler). Stir until smooth. Let it cool for 2–3 minutes so it doesn’t scramble the eggs.

Whisk in sweetener. Add the erythritol or allulose and whisk until glossy. This helps dissolve the sweetener and gives a shinier top.

Add eggs and vanilla. Whisk in the eggs one at a time, then the vanilla. Mix just until combined. The batter should look thick and shiny.

Combine dry ingredients. In a separate bowl, whisk the almond flour, cocoa powder, baking powder, and salt. Break up any clumps for a smooth batter.

Bring it together. Fold the dry mixture into the wet mixture with a spatula. Stir until no streaks remain. If using chocolate chips, fold them in now.

Swirl the peanut butter. Pour the batter into the lined pan and smooth the top. Warm the peanut butter slightly so it’s pourable. Dollop it over the batter in spoonfuls and use a knife or skewer to create swirls. Don’t overmix—distinct ribbons look and taste best.

Bake. Bake for 18–24 minutes, depending on your oven. The center should be just set with a slight jiggle, and a toothpick should come out with moist crumbs (not wet batter). Slightly underbaked is better for fudgy brownies.

Cool completely. Let the pan cool on a rack. For the cleanest slices and dense texture, chill for 30–60 minutes before cutting.

Slice and serve. Lift out with the parchment and cut into 12 squares. Sprinkle with flaky salt if you like.