In a small saucepan over medium heat, melt the butter. Continue cooking until it turns golden brown and smells nutty. Remove from heat and let cool for about 10 minutes.

Combine the chopped chocolate with the cooled browned butter and stir until smooth. Let it cool slightly.

In a bowl, whisk together the all-purpose flour, bread flour, cocoa powder, baking powder, baking soda, salt, and espresso powder.

In a separate large bowl, beat the granulated sugar, brown sugar, and eggs until slightly lightened and smooth. Mix in the vanilla.

Add the melted chocolate mixture and stir until combined.

Gradually add the dry ingredients, mixing just until incorporated.

Cover and chill the dough for 30 minutes.

Preheat oven to 350°F and line baking sheets with parchment paper.

Scoop about 2 tablespoons of dough per cookie and place 2 inches apart.

Bake for 8–10 minutes, until the edges are set but the centers are still soft.

Let cool on the baking sheet for 10 minutes, then transfer to a rack to cool completely.

Beat the softened butter and brown sugar until light and fluffy.

Add the powdered sugar and heat-treated flour gradually, mixing until smooth.

Mix in the vanilla and salt.

Add heavy cream, 1 tablespoon at a time, until the filling is soft, fluffy, and spreadable.

Pair cookies by similar size.

Spread or pipe a generous layer of cookie dough buttercream onto the flat side of one cookie, then top with another cookie.

Gently press together.