Adjust oven rack to middle position and preheat oven to 375°F. Spray standard muffin tin with non-stick pan spray or use muffin liners.

Sift flour, cocoa powder, instant coffee granules, baking powder, baking soda, and salt into a large bowl; set aside.

In a medium bowl, whisk sugar with eggs until light, about 30 seconds. Whisk in oil, milk, espresso or coffee, vanilla, and bitters until combined.

Whisk wet mixture into dry mixture until just combined. Do not overmix. Stir in chocolate chips or chunks and walnuts.

Evenly divide batter between muffin cups.

Combine the topping ingredients in a small bowl and sprinkle evenly over the muffins.

Bake until set, 18 to 20 minutes. Let muffins cool in pan for 15 minutes, then remove muffins from tin and place on wire rack to cool.