Preheat the oven to 350F.

Butter and line a 9-by-13-inch metal baking dish with parchment paper

In a small saucepan, melt the butter over low heat.

Remove from the heat and add the chocolate. If the heat from the butter does not fully melt the chocolate, put the pan back over the heat for 10 seconds and stir until melted.(note: if you are using a chocolate bar, you will need to melt the chocolate longer on the stove)

Set chocolate mixture aside to cool.

Sift the flour, espresso powder and cocoa powder into a small mixing bowl. Set aside.

In a medium mixing bowl, combine the eggs, sugars, salt, and vanilla. Using a mixer fitted with the whisk attachment (or hand mixer), beat on high speed until the mixture thickens and becomes pale in color and falls from the beater in a wide ribbon that folds back on itself and slowly dissolves on the surface, 4 to 5 minutes.

Using a rubber spatula, fold the cooled chocolate into the egg mixture.

Add the flour, cocoa powder, and espresso powder and fold it in quickly but gently with the rubber spatula so that you don't deflate the air that's been incorporated into the eggs.

Pour half of the batter into the prepared dish and smooth the top with a spatula. Place thin mint cookies in an even layer across the batter. Pour the second half of the batter into the prepared dish and smooth the top with a spatula

Let cool completely on a wire rack.

Using a sharp knife, cut into 12 squares, or size desired.