Set your oven to 150°C Fan/170°C/340°F/Gas mark 3
If using a Bundt tin, grease the tin by spreading softened butter over all of the insides of the tin using your pastry brush, ensure you get into all of the crevices. Then sprinkle over 2 tablespoons of flour and shake the tin to ensure it lightly covers all of the insides. Shake out any excess. If you are using sandwich tins, grease the tins and line the bases with greaseproof paper.
Take your butter and sugar and beat together until they are soft, lighter in colour and have an easily spreadable consistency.
Wipe the bowl down with your spatula to ensure all of the butter and sugar are mixed in.
Sift together your flour, baking powder and salt into your other bowl.
To the butter and sugar mixture, add one egg and a heaped dessert spoonful of flour and beat on a low setting until just combined.
Repeat another 3 times until all of your eggs have been used.
Again wipe down the bowl with your spatula to ensure all of the mixture is incorporated.
Add your vanilla essence, lemon zest and lemon juice and mix until just combined.
Mix together your sour cream and milk separately.
Add ¼ of your milk mixture to your batter and mix on a low setting until just combined.
Add a heaped dessertspoonful of flour and mix in on low until just combined.
Repeat the above two steps until all of your milk mixture and flour mixture have been mixed in.
Again wipe down the sides and base of your bowl with your spatula to ensure that everything is totally incorporated.
Set your mixer to a high setting and give the batter a fast blast for about 10 seconds.
Tip the mixture into your Bundt tin, or divide equally between your sandwich tins
Take your blackberries and pop them in the top of the batter, spaced apart and just pushing down slightly so mostly submerged. They will continue to sink as they bake.
Bang the tin down on your work surface a couple of times to release any large air bubbles in the batter.
Bake in the centre of your oven for about 55-60 minutes, until a skewer or cocktail stick inserted into the centre comes out clean.
Leave to cool in your tin for 10 minutes before inverting onto a cooling rack to cool completely.