In a mixing bowl, pour in 1 cup of warm water, and sprinkle 2 teaspoons of instant yeast along with 1 tablespoon of sugar. Stir gently and let it sit for about 5 minutes until frothy. Then, gradually add 3 cups of all-purpose flour and 2 tablespoons of cooking oil, mixing until a shaggy dough begins to form.

Transfer the dough to a floured surface and knead for 5 to 8 minutes until smooth and elastic. Form the kneaded dough into a ball and place it in a lightly oiled bowl, covering it with a damp cloth.

After the dough has risen for about 1 hour, punch it down to release air bubbles. Divide it into two portions. Flatten one piece and incorporate cocoa into the other.

In a separate bowl, mix 1/4 cup of cocoa powder with the second portion of dough. Knead until uniform in color, then roll it into a rectangle that matches the plain dough.

Stack the plain dough on top of the chocolate dough, gently pressing them together. Roll tightly into a log and cut into 6 to 8 equal portions.

Place each cut piece on a square of parchment paper, spaced out for growth. Proof in a warm environment for 30 to 35 minutes.

Prepare a steamer with water boiling gently. Arrange the buns in the steamer and cover with a lid, leaving a slight gap. Steam for about 18 minutes.

Allow the buns to cool for 5 minutes in the steamer before serving warm with optional toppings.