Step 1

Preheat oven to 350F Grease and flour two 9-inch round cake pans, putting a round of parchment paper at the bottom of each one.

Step 2

Whisk together the flour, baking powder and salt and set aside.

Step 3

Put the sugar and lemon zest into a food processor and process until moist, pale yellow, and no large pieces of zest remain. This will take under a minute.

Step 4

Cream the butter and sugar together in a stand mixer for 3 minutes, scraping down the sides of the machine as necessary.

Step 5

Beat in the eggs, one at a time, scraping down the sides of the bowl in between each addition. Beat in the lemon paste, if you're using it.

Step 6

Turn the mixer to low and add the flour mixture, and just before it's completely mixed in, add the buttermilk and lemon juice. Blend until combined, but don't over-mix. I like to finish by hand to get everything incorporated.

Step 7

Gently fold in the blueberries. Note: I didn't use quite the whole pint. Then divide the batter equally between the two prepared pans. Spread out to an even layer.

Step 8

Bake the cakes in the center of the oven for about 35 minutes, or until just beginning to pull away from the sides of the pan, the top springs back when touched, and a toothpick comes out without any wet batter on it (moist crumbs are fine.)

Step 9

Let the cakes cool on a rack for 10 minutes, then remove them from the pans and continue to cool completely before frosting.

Step 10

To make the frosting, beat the soft butter and sugar together, gradually adding enough lemon juice to create a creamy spreadable frosting. If it is too thick, add more lemon juice, and if it is too thin, add a little more sifted sugar.

Step 11

Frost the cake and then decorate as you like.