In a mug combine buttermilk with vinegar and stir.
In a large mixing bowl combine all purpose flour, spelt flour, beetroot powder and baking soda. Whisk very well to combine.
Add the eggs and the oil and start mixing with a wooden spoon, until absorbed. Gradually add the buttermilk with vinegar and keep stirring When a dough forms, knead gently with your hands for 2-3 minutes, until all ingredients are absorbed and the dough is not too sticky any more.
In a bowl combine melted butter, coconut sugar and cinnamon. Whisk very well until smooth.
Preheat oven to 360 degrees F/ 180 degrees C.
Divide the dough into three parts. Place one part between two sheets of parchment paper and sprinkle it with flour. Roll out the dough into a thin, more or less rectangular sheet. Spread on top the filling and roll up the sheet, starting from the long side end. Cut the resulting long roll into 4 equal parts. Arrange them on a greased pan (9 x 7 inch/ 23 x 20 cm or similar). Repeat the procedure two more times with the rest of the dough and the filling.
Bake the rolls for about 30 minutes or until the tops start becoming golden. Frost when still hot or warm.
In a medium bowl combine all the ingredients for the frosting. Whisk until smooth. Spread the frosting on top of the rolls.
Keep the leftover rolls (frosted or unfrosted) in the fridge, covered with cling film. Reheat in the microwave or in a hot oven before serving.
The buttermilk can be substituted by full fat plain yogurt mixed with water 1:1.
The quantity of the buttermilk is approximate - you might need a bit more or less. For that reason I recommend to be added gradually at the end. In case you don't have more buttermilk but the dough is still too thick, add some water or yogurt instead.
You can skip the beetroot powder if you don't have it. Beware that this might change the proportion of the buttermilk.
This is a healthy recipe with no refined sugar. If you are not concerned with nutrition, you can easily substitute the coconut sugar by brown or white sugar and the maple syrup by powdered sugar.