Lemon cake with blackberry frosting

After several failures with macarons, I decided to bake cupcakes again. It's more of a rewarding experience coz cupcakes never really fail. It's spring time so I'm excited to try Annie's recipe of the lemon cupcake. The cakes turned out lemony and fluffy. For the blackberry frosting, it is nice to have the pinkish color from the blackberry puree without having to add food coloring, however the frosting is too sweet for my liking. I couldn't really taste the tartness of the blackberry at all! I wonder how to get the right consistency without having to add so much icing sugar.

Make 15 cupcakes

For the lemon cake
1.5 cups cake flour
1/2 tbsp baking powder
1/4 tsp salt
115g unsalted butter, at room temp
1/2 lemon, zested
1 cup sugar
3 large eggs, at room temp
2/3 cup buttermilk, at room temp (1tbsp lemon juice & just under 1 cup milk, sit for 10mins)
1/2 tbsp lemon extract
1/2 tsp vanilla extract

For the blackberry frosting
115g unsalted butter, at room temp
2 cups icing sugar, sifted
1/4 cup blackberries
1/2 tsp vanilla extract
pinch of salt
blackberries, for garnish

Directions

1. Preheat oven to 175C. Line cupcake tins with paper liners, set aside.
2. In a medium bowl, combine the cake flour, baking powder and salt. Whisk to combine. In another bowl, add the butter and lemon zest. Beat on medium high speed for about 3 mins, until light and fluffy. Scrape down the sides of the bowl and beat for 1 more min.
3. Add sugar to the butter mixture, 1/4 cup at a time, beating 1 min after each addition.
4. Mix in the eggs one at a time until incorporated. Scrape down the sides of the bowl after each addition. 
5. Combine the buttermilk and the lemon extract and vanilla extract with a cup. With the mixer on low speed, add the dry ingredients alternately with the wet ingredients, beginning and ending with the dry ingredients and mixing just until incorporated. Scrape down the sides of the bowl and mix for 15 seconds.
6. Fill the cupcake liners 2/3 of the way full with batter. Bake until golden and a toothpick inserted into the center comes out clean, about 18mins.