Preheat oven to 325°F (163°C). Line a baking sheet with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In a large bowl, beat the butter (or oil) and sugar together until well blended. Beat in the eggs one at a time, then stir in the vanilla and espresso powder.

Gradually add the dry ingredients to the wet ingredients. Mix just until combined. If the dough feels very stiff or dry, mix in 1 to 2 tablespoons of milk or cream to bring it together.

Stir in the walnuts and chocolate chips.

Divide the dough in half. On a lightly floured surface (or directly on the parchment), shape each half into a log about 11 inches long and 2 inches wide. Slightly flatten the tops. Dampen your hands with water to smooth and shape the logs neatly.

Brush the tops and sides with beaten egg for a glossy finish.

Chill the logs on the baking sheet for 15–30 minutes (optional but helps shape).

Bake for 25–30 minutes, until the logs are set and just firm to the touch. Cool on the baking sheet for 10 minutes.

Reduce oven temperature to 300°F (149°C).

Using a serrated knife, cut the logs diagonally into ½-inch slices. Arrange the slices cut-side down on the baking sheet.