Preheat oven to 350 °F (175 °C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang.

In a bowl, whisk melted butter with granulated and brown sugars. Beat in eggs one at a time, then stir in vanilla extract and Greek yogurt.

In another bowl, sift together almond flour, cocoa powder, tapioca starch, xanthan gum, salt, and baking powder.

Gently fold dry ingredients into wet mixture until just combined. Stir in espresso powder if using. Let batter rest for 10 minutes to hydrate the flour.

Fold in chocolate chips or nuts as desired.

Spread the batter evenly in the prepared pan.

Bake for about 30 minutes, until the top has a shiny, crackly crust and a toothpick inserted into the center comes out with moist crumbs.

Cool completely in the pan on a wire rack (about 1 hour).

For best texture, let brownies rest sealed for a day or refrigerate overnight to deepen flavor and fudginess.

Lift brownies out using the parchment overhang. Slice into 12 pieces and serve.