Preheat the oven to 350°F and line an 8×8-inch baking dish with parchment paper or spray it with nonstick spray.

In a large mixing bowl, whisk the eggs, melted butter, and vanilla extract until smooth.

Add the sugar substitute and whisk again until well combined.

Stir in the almond flour, cocoa powder, and baking powder until the batter is evenly mixed.

Fold in walnuts or chocolate chips if using.

Pour the batter into the prepared baking dish and spread it evenly.

Bake for about 25 minutes, until the edges are set and a toothpick inserted in the center comes out mostly clean.

Allow the brownies to cool for about 10 minutes before cutting into squares and serving.