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Heat the oven to 190C/170C fan/gas 5. Use a pastry brush to brush two 20cm round cake tins with a little butter, then line them with baking parchment (see How to Line a Cake Tin).
Place a clean cloth under a large bowl – this will keep it steady as you mix. Weigh the butter, caster sugar and vanilla extract into the bowl.
Make sure the whisk attachments are fitted securely in your hand whisk, then plug it in. Turn the whisk on to a medium speed and use it to whisk the ingredients together for 2 mins, or until they look smooth and creamy. Make sure you keep your fingers away from the whisk while it’s turned on.
Crack the eggs into a small bowl. Continue whisking the butter and sugar and use your other hand to tip in the eggs, one at a time. Mix for 1 min more or until smooth.
Weigh the flour and add it to the bowl. Use a spatula to fold the ingredients together until there are no pockets of flour and the cake mixture is smooth.
Use your spatula to divide the cake mixture evenly between the cake tins, then spread the mixture to the edges and smooth the surface.
Use oven gloves to place the tins on the middle shelf of the oven and set a timer for 22 mins. When the timer beeps, use your oven gloves to pull the oven rack out of the oven slightly but leave the cakes inside, they should be risen and golden brown. Insert a skewer into the centre of the cakes to check if they’re cooked: if the skewer comes out dry, they’re cooked; if there is any wet cake mixture on the skewer, the cakes need a little longer in the oven. If this is the case, return them to the oven and bake for another 5 mins, then check again.
Use your oven gloves to carefully remove the cakes from the oven, place them on a cooling rack and allow to cool for 15 mins in the tin. When cool enough to handle, turn the cakes out onto the rack and remove the baking parchment, then leave until completely cool.
Measure the double cream in a jug and pour it into a large mixing bowl. Measure the icing sugar in a measuring spoon and add it to the cream. Use your electric hand whisk to whip the cream until it is softly whipped, like clouds.
Place one of the cakes on a plate or cake stand, then use a spatula or palette knife to spread the cream over the cake in an even layer. Put the blackberries on top of the cream, then put the other cake on top. Use a sieve or icing shaker to dust the cake with a little icing sugar. This is best served straight away but will keep in the fridge for two days.