Gather all ingredients.

Combine butter, light brown sugar, and white sugar in a bowl and beat with an electric mixer until fluffy, 2 to 3 minutes. Beat in egg yolks and orange zest.

Combine flour and buckwheat flour in a small bowl. Stir flour mixture into butter mixture until you have a soft dough.

Divide the dough in halves and set on a sheet of parchment paper or plastic wrap. Shape into 2 logs, about 2 inches in diameter, using the paper or wrap, and wrap up tightly. Refrigerate until firm, at least 6 hours.

Preheat the oven to 350 degrees F (180 degrees C). Line 2 baking sheets with parchment paper. Unwrap dough logs and brush each log with beaten egg.

Pour demerara sugar into a shallow dish and roll the egg-washed logs in the sugar, until encrusted all around with sugar.

Cut each log into 1/2-inch slices and arrange on the prepared baking sheets. Sprinkle with buckwheat groats if you like.