Heat the oven to 350°F (175°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper.

Whisk the flour, baking powder, and salt together in a medium bowl. Set the dry ingredients aside while you prepare the lemon-scented sugar.

Rub the finely grated lemon zest into the granulated sugar with your fingertips until the sugar feels slightly moist and smells intensely citrusy. This releases the natural lemon oils and gives the cake a stronger flavor.

Beat the softened butter and lemon-scented sugar for 3–4 minutes, until pale and fluffy. Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.

Add the dry ingredients in three additions, alternating with the room-temperature buttermilk. Mix only until the batter is combined, then stir in the lemon juice at the end without overmixing.

Divide the batter evenly between the prepared pans and smooth the tops. Bake for 22–28 minutes, until the centers spring back lightly and a toothpick comes out clean or with a few moist crumbs.

Let the cakes cool in their pans for about 10 minutes, then turn them onto wire racks. Cool completely before adding syrup or frosting so the buttercream does not melt.

For the optional syrup, heat the lemon juice and sugar in a small saucepan until the sugar dissolves. Brush or spoon the warm syrup over the cooled cake layers and allow it to absorb.

For the buttercream, beat the softened butter until smooth. Gradually add the sifted confectioners’ sugar, then beat in the lemon juice, zest, salt, and enough cream or milk to create a smooth, spreadable consistency.

Place one cake layer on a serving plate and spread it with frosting. Add the second layer, then frost the top and sides before slicing and serving.