Make the chia or flaxseed "egg" by combining 1 tablespoon of chia seeds with 5 tablespoons of water. Let sit for 10-15 minutes.
Preheat the oven to 150° C / 302° F (fan on).
Combine all the dry ingredients.
Mix the apple cider vinegar with the oil and the water, and combine with the dry ones.
Line the muffin tins with the papers liners.
Distribute the batter evenly across the muffin tins.
Distribute 125 g / 1,2 cups raspberries across the batter in the muffin tins. Use your fingers or a teaspoon to push the raspberries into the batter and cover them slightly with batter.
Bake for 20-25 minutes at 150° C / 302° F (fan on). Leave to cool completely.
For the icing, grind the sugar in a coffee or nut grinder until it has the consistency of icing sugar. You can skip this step if you're using regular icing sugar.
In a bowl, mash the raspberries with a fork or spoon. For a seedless raspberries purée, strain the mashed raspberries through a fine sieve (until only the seeds are left, that is, you want to make sure that no drop of raspberry juice is wasted).
Combine the icing sugar with the raspberry purée. The icing should be rather thick, not runny.
Decorate the muffins immediately with the icing, as the icing will harden up quickly.
Press a hazelnut into the icing of each muffin.
Let sit for about 2 hours for the icing to completely harden up before serving.
The muffins will keep in an airtight container for a couple of days. I recommend serving them within 2-3 days, as the icing will start softening and losing its bright pink color after 3 days.