Heat oven to 200C/180C fan/gas 6 and line a 12-hole muffin tin with paper cases. Put all the topping ingredients, apart from the jam, in a bowl and rub together.
Beat the butter and caster sugar together until pale and fluffy. Add the eggs and beat in for 1 min, then mix in the yogurt, extracts and milk. Combine the flour, baking powder and bicarb in a bowl with 1/4 tsp fine salt, then tip this into the wet ingredients and stir in. Finally, fold in the blueberries and divide the mixture between the muffin cases. Top each muffin with 1 tsp blueberry jam, then scatter over the crumble mixture.
Bake for 5 mins, then reduce oven to 180C/160C fan/gas 4 and bake for 15-18 mins more until risen and golden, and a cocktail stick inserted in comes out with just jam on it – no wet cake mixture.
Cool in the tin for 10 mins, then carefully lift out onto a wire rack to finish cooling. Will keep for 3-4 days in an airtight container – after a day or two, pop them in the microwave for 10-15 secs on High to freshen up.