Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper and set it aside. This ensures a non-stick environment for our cookies.

In a mixing bowl, whisk together the all-purpose flour, buckwheat flour, baking powder, baking soda, and salt. Mixing these together well avoids any clumps and ensures even distribution throughout the dough.

In a separate large bowl, combine the melted butter with both sugars. Mix until the mixture is smooth and creamy. This process should take about 2-3 minutes. The butter and sugar create the foundation for our cookies' sweetness and texture.

Now, whisk in the large egg and egg yolk until fully incorporated. Follow it up with the pure vanilla extract. The combination of eggs helps bind everything while also providing a rich flavor.

Gradually add the dry flour mixture to the wet ingredients. Stir gently until just combined. Avoid over-mixing. It’s okay if a few dry flecks remain.

Time to fold in the star players! Add the chopped chocolate, nuts, toasted buckwheat groats, and, if you’re using it, the espresso powder. Mix these in thoroughly, ensuring every bite has a bit of chocolate and crunch.

Using a cookie scoop or tablespoon, drop rounded portions of dough onto your prepared baking sheet. Leave enough space between each scoop—these cookies will spread out nicely while baking.

Sprinkle a touch of flaky sea salt on each cookie bite before placing them in the oven. This small finishing touch enhances the flavors substantially.

Bake for 12-14 minutes, or until the edges are golden brown. The centers should be soft but set. Don’t worry if they look slightly underbaked; they’ll continue to firm up as they cool.

Once done, remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to wire racks to cool completely. You can’t rush an excellent cookie!