Make the jam first - ideally at least a day before so that it can cool and set
Put the blackberries, syrup and orange juice in a saucepan and bubble gently for 10 minutes, until you can squish the blackberries with the back of a spoon
Squish until you don't have any large whole blackberries left then stir in the chia seeds, cook for 1 minute then cool and transfer to a storage container (see note)
Preheat your oven to 180 fan / 200 C / 400 F / Gas 6 and line a 12 hole muffin tin with muffin cases
To make the muffins whisk together the milk and vinegar in a small bowl or jug and set aside to thicken while you weight out the dry ingredients
In a large bowl put the flour, sugar, bicarb and spices and stir together
Add the oil to the milk then pour all that into the bowl of dry ingredients, mixing well and making sure there are no flour 'pockets' left
Divide the mixture between the 12 muffin cases, filling each one roughly 2/3 full
Add a teaspoon of jam to each of the muffins then gently swirl the jam into the batter using something pointed like a cocktail stick or skewer
Bake for 20-25 minutes until well risen and golden and firm to the touch
Allow to cool in the tin for a few minutes before lifting out onto a wire rack to cool completely
Store in an airtight container at room temperature or freeze any leftovers
Notes
Sub the agave for maple syrup or caster sugar - if using sugar you may need to add a little more as agave is sweeter than sugar
If making the jam well in advance, store in a sterilised, jar with a tight fitting lid, otherwise any lidded container will do. Keep in the fridge until using
Recipe by The Cook & Him at https://thecookandhim.com/recipes/blackberry-jam-breakfast-muffins/