01 -
Preheat your oven to 350°F. Grease two 9-inch round cake pans and line them with parchment paper, then grease again. Beat the butter and sugar together in a large bowl until fluffy, about 2-3 minutes.
02 -
In a small bowl, whisk together the cake flour, salt, and baking powder.
03 -
Measure out the milk and add the almond extract, stirring gently.
04 -
Alternate adding the dry ingredients and the milk mixture to the butter and sugar mixture, a little at a time.
05 -
In a clean bowl, beat the egg whites on high until stiff peaks form. Fold them gently into the batter, being careful not to over-mix.
06 -
Pour the batter evenly into the prepared cake pans and bake for 24-27 minutes. The cakes should be light golden and bounce back when pressed gently. Allow them to cool for 10-15 minutes, then transfer to a cooling rack.
07 -
Mix cold water, lemon juice, and cornstarch in a small bowl. Put blackberries and sugar in a saucepan, pour the cornstarch mixture over the top, and heat on medium-low. Bring to a boil, stirring constantly, and let it boil for 2-4 minutes until thickened. Strain through a fine mesh sieve to remove seeds, if desired.
08 -
Blend blackberries and lemon juice in a blender for 30-60 seconds. Mix the blackberry mixture with 2 cups of powdered sugar. Whip the room temperature butter until light and fluffy, then beat in the remaining powdered sugar. Add the blackberry mixture gradually to prevent the butter from seizing. Chill for 30 minutes.
09 -
Place a small dollop of frosting on a cake stand or plate. Put the base layer of cake on top and spread the blackberry filling over it. Place the second cake layer face down on top. Frost and decorate the cake as desired.