• Preparing the dough:
• Before you start, make sure all the ingredients are at room temperature.
• Whisk the softened butter and sugar until they become fluffy. Add the eggs and thoroughly blend them in. Combine the baker's ammonia and milk, then add this mixture to the egg mixture.
• In a separate bowl, mix the flour and grated lemon zest together. Gradually add this mixture to the egg mixture. The resulting dough should appear somewhat oily and should not stick to your hands. If needed, you can incorporate an extra tablespoon of flour.
• Place the mixture in the fridge for a few minutes while you prepare for the next steps.
• Preheat your oven to 360°F (180°C). Prepare a quarter-sized cookie sheet pan or a similar-sized baking pan.
• Grease the outside bottom of the pan generously with butter and dust it with flour. This step is necessary only before the first layer.
• You'll be creating six layers, possibly seven if you gather enough small dough scraps from the initial six layers.
• Take the dough from the fridge and place it on a floured work surface. Knead the dough again and divide it into six equal pieces, placing the rest in separate ziplock bags.
• Roll out each piece thinly, less than 1/8-inch thick, ideally forming a rectangular shape close to the dimensions of your pan. This may take a little practice.
• Carefully transfer each rolled-out dough layer onto the cookie sheet pan, then trim the edges with a small knife to achieve an almost perfect rectangular layer of dough. Save any extra dough scraps.
• Bake each layer for 10 to 15 minutes until the edges barely darken.
• After baking, designate an area where you will stack your cake layers. You'll need a large surface to press them with a heavy board until they cool completely, preferably overnight.
• Begin preparing the filling after your cake layers have completely cooled, preferably on the same day or even the following day.

Prepare the filling

• In a saucepan, combine the sugar and 4 cups of milk. Heat on medium until it reaches a soft boil.
• In a separate container, mix the remaining 1/4 cup of milk with the cornstarch. Add this mixture to the boiling milk and sugar, then reduce the heat. Continuously stir and cook until the mixture thickens. Note that it will thicken further upon cooling.
• Remove the mixture from heat, transfer it to a bowl, and continue stirring while it cools. You may want some assistance from others, as it can be tiring on your arms.
• After the mixture has cooled, add the freshly squeezed lemon juice and thoroughly blend it in. Start by adding around 8 tablespoons, and then continue to add more until it reaches your desired level of tartness.
• The filling must be entirely cooled before adding the butter.
• When ready, soften and cream the butter, then gradually incorporate it into the cooled cream while mixing. A stand-up mixer can make this process easier. Taste the buttercream again and add more lemon juice if you like it.

Assembling the Cake:

• Place the first cake layer on a board or a cookie sheet and spread about 5 spoonfuls of filling evenly over it. The filling should have a similar thickness to the cake layer, so you'll need to estimate.
• Add the next cake layer and repeat the process, alternating between layers and filling until the top layer is placed.
• Place the board that was used for pressing the cake layers back on top. Leave it for at least five hours, preferably overnight.
• To finish the cake, dust it with powdered sugar.
• Before cutting the whole cake into approximately 2x2-inch (or slightly smaller) cubes, trim the edges thinly to remove any imperfections. Enjoy!