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1
Preheat oven to 350u00b0. Grease a 9x9 square baking pan.
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2
Mix together dry flour, baking powder, baking soda, and salt in a small bowl.
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3
Add softened butter and sugar to a medium bowl. Using an electric mixer or a whisk, cream the butter and sugar until completely combined.
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4
Add the eggs one at a time, mixing after each addition. Add the buttermilk and flour mixture in three parts, mixing after each addition. Add vanilla and lemon zest with the final buttermilk addition.
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5
Scrape the batter into the prepared pan and use a spatula to make sure it's distributed evenly. Bake for 12 minutes, or until a toothpick inserted into the center comes out clean. Let cool at least 15 minutes in the pan. The cake will pull away from the sides as it cools.
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6
Add the mascarpone, sugar, vanilla, lemon juice, and milk to a bowl. Using an electric hand beater or stand mixer with the whisk attachment, beat all ingredients together until smooth and fluffy, about 2 minutes. Scrape into a pastry bag or the corner of a gallon ziploc with the tip cut off.
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7
Cut the cake in half to create two rectangles and trim the edges so all sides are exposed, just like how you would cut the crusts off of toast.
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8
Place the first layer on your serving platter. Using the pastry bag, pipe dots or a ruffled line around the perimeter of the cake to create a decorative edge. Fill in the center with a thin layer of the filling. Place rows of blackberries across the short way, leaving a little space in between each row to make it easier to cut slices later.
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9
Place the second layer on top and, following the same steps for decorating, pipe a decorative perimeter and place rows of blackberries. Pipe dots in between the blackberries to fill in the top.
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10
Refrigerate if not serving right away. This cake should hold up well in the fridge, although I can only attest to one day, as that's how long it lasted in my house.
