Begin by preheating your oven to 375 F or 190 C. Lightly grease a 9 x 13 inch cake or jelly roll pan and line the bottom with parchment or wax paper. This step ensures the sponge releases easily after baking.

In a medium bowl, sift together the flour, baking soda, and salt. Set aside. Prepare a clean, lint free kitchen towel and dust it generously with powdered sugar. This prevents the warm cake from sticking while rolling.

Zest and juice one lemon, setting aside about 3 tablespoons of juice and zest combined. In a large bowl, whisk the eggs and sugar together until the mixture becomes pale yellow, triples in volume, and forms thick ribbons when lifted.

Gently sift the dry ingredients over the egg mixture and fold carefully using a spatula. Take care not to deflate the batter. Fold in 2 tablespoons of the lemon juice and zest until just combined.

Pour the batter into the prepared pan and spread it evenly with an offset spatula. Bake for 8 to 10 minutes, or until the cake springs back lightly when touched and the sides begin to pull away from the pan. The center should reach an internal temperature of 205 F or 96 C.

Carefully invert the hot cake directly onto the prepared powdered sugar towel. Gently peel away the parchment paper. Starting from the short end, roll the cake up with the towel while it is still warm. Allow it to cool completely on a wire rack.

To make the filling, beat the softened butter until smooth and creamy. Gradually add powdered sugar on low speed until fully incorporated. Beat in the remaining lemon juice and zest until light and fluffy.

Once the cake has cooled, gently unroll it. Spread the lemon buttercream evenly across the surface. Roll the cake back up carefully without the towel. Cover and refrigerate for 1 to 2 hours to allow the filling to firm up before slicing.

Before serving, let the cake sit at room temperature for about 30 minutes. Dust the top with powdered sugar if desired.