Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper.
In a small bowl, dissolve the instant espresso powder in 1 teaspoon of hot water and stir until smooth. Set aside.
In a large bowl, whisk together the melted butter and golden monk fruit sweetener. Add the eggs, vanilla extract, and dissolved espresso. Whisk until everything is smooth and well combined, with a rich coffee color throughout.
Add the almond flour, baking powder, and salt. Stir with a spatula until a thick batter forms. Fold in about two-thirds of the chopped dark chocolate.
Spread the batter into the prepared pan and smooth the top. Press the remaining chopped chocolate pieces into the surface of the batter so they are visible on top.
Bake for 22 to 26 minutes, until the edges are golden brown and set, and a toothpick inserted in the center comes out with just a few moist crumbs.
Sprinkle flaky sea salt over the top immediately after removing from the oven. Let cool completely in the pan before slicing into 16 squares. The espresso flavor deepens as they cool.