Make the cake: Heat oven to 350F. Grease a 9x13-inch baking pan or alternately, line with parchment if you'd like to be able to remove the cake from the pan to serve. Set aside.
In a 2-cup measuring cup, combine the milk and lemon juice. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In a large bowl with an electric mixer or the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar and lemon zest until light and fluffy, about 5 minutes. Add the eggs, one at a time, mixing for about 30 seconds after each addition. Scrape down the bottom and sides of the bowl to ensure the mixture is well combined.
With the mixer on low speed, alternately add the flour mixture and the milk mixture to the bowl in three additions (1/3 of the flour, 1/2 of the milk, 1/3 of the flour, remaining milk, remaining flour). Mix until just combined, then use a rubber spatula to finish mixing the batter, to ensure all the ingredients are well incorporated.
Pour the batter into the prepared pan. Smooth the top and tap the pan on the counter a few times to release any large air pockets.
Bake for 35 to 40 minutes or until puffed and golden and a cake tester inserted into the centre of the cake comes out clean. Cool the cake completely in the pan, on a rack before frosting.
Prepare the frosting: With an electric mixer or stand mixer with a paddle attachment, beat the room temperature butter for 1 minute. Add the icing sugar, lemon zest, lemon juice and pinch of salt to the bowl. With mixer on low, mix until icing sugar is combined, then increase mixer to medium high and beat until light and fluffy. If necessary add a teaspoon or two of milk to thin, if frosting seems too thick. Alternately, add a bit more icing sugar if icing seems to thin. Frosting should be smooth, shiny and hold it's shape.
Spoon the frosting in 5 or 6 small mounds on top of the cooled cake. Use a spatula to gently spread the frosting out to the edges. Smooth or swirl frosting. Sprinkle with decorations, if desired.
Frosted cake will keep well, covered for up to 2 days. The cake can also be made ahead and frozen, either just the cake or the frosted cake. Save any decorations until after freezing.
Be sure to read the "Cook's Notes" in the original post, where I share some valuable tips, options, substitutions and variations for this recipe!