Preheat oven to 360º F/ 180º C.

Grease a small to medium bundt pan: pour 1-2 tablespoon oil and ½ teaspoon all-purpose flour inside and spread them carefully everywhere with a kitchen brush.

Sift the flour (optional but recommended).

In a large mixing bowl beat the eggs with an electric mixer for at least 5 minutes, until the mixture increases its size and becomes pale.
Add the oil and vanilla. Beat again shortly to combine.
In a small bowl or a mug mix the baking soda into the yogurt. Add them to the batter and mix well to combine. 
Gradually fold the flour into the batter using a rubber spatula. Fold gently, until no flour lumps remain. Alternate with the grapefruit juice. Mix gently until the batter is smooth and both ingredients are absorbed.

Transfer the batter into the prepared pan. Bake for about 40 minutes or until the top is golden-brown and a wooden skewer inserted in several places comes out clean.

Let the cake cool completely before removing it from the pan.

Carefully run a knife around the edges and then invert it on top of a serving plate.

Pour the glaze on top before serving.

Melt the chocolate on a double boiler. Add the grapefruit juice and stir to combine.

When the mixture is smooth remove from the heat and pour on top of the cake.