Preheat the oven to 190℃
Grease and line the base of the cake tin
First make the streusel - rub the cold margarine/butter into the flour and sugar to make a crumbly craggy mix. Set aside.
Next make the muffin mix. Mix together the flour, caster sugar, baking powder, bicarbonate of soda and salt.
Add the blackberries and toss in the flour mix, as this helps them stay suspended in the batter and not sink to the bottom.
Mix together the milk, melted margarine and lemon juice and pour into the dry mix. Gently mix so the batter is just combined but with no patches of raw flour.
Pour into the cake tin and level off. You can add a few more blackberries at this stage if you like your muffins extra fruity.
Scatter over the streusel mix
Bake for 30-35 minutes until it's golden and risen and a knife comes out clean of batter
Remove from the tin and let cool on a wire rack
This muffin cake is best enjoyed on the day it is made or the next day
