Butter a banana loaf pan (4.5” by 8.5”) and preheat the oven to 400F.
Wash and roast the beets until a knife can pierce them easily, about 45 minutes for a plum sized beet. It’s best to rub the peels off while they are hot- if you can. Otherwise, peel them once they are cool, then grate them using a box grater. You can always boil beets to save time, but roasting will give you a richer flavour. Once the beets are cooked, lower the oven to 350F.
Sift the flour, baking powder, cocoa powder and salt together. Set aside.
Put chocolate to melt in a bain marie: let the chocolate melt in a bowl that is resting in a pot of hot water for gentle heat.
Add the butter and the hot coffee to the melted chocolate and stir until the mixture is melted and homogenous.
Whisk the egg yolks with one quarter of the sugar until dissolved, then whisk in the sour cream. Then fold in the chocolate butter mixture, then fold in the flour mixture. Then fold in the grated beets.
Using a standing mixer, whisk the egg whites and the remainder of the sugar on medium speed using the whisk attachment until you have billowy soft to medium peaks.
Gently fold in one third of the egg white mixture into the batter, then fold the batter back into the remaining egg white mixture.
The mixture should fill the banana loaf pans three quarters full. Bake for 40 minutes and let cool on a cooling rack.
Once cool, ice the cake with crème fraiche and serve.