Preheat the oven to 350ºF. Use the 1 tablespoon of butter to grease the insides of a 6-cup jumbo muffin tin. Flour the tin and set aside

In a medium bowl, whisk together the flour, baking powder and salt. In a small bowl or liquid measuring cup, mix together the buttermilk, vanilla, lemon zest and lemon juice.

In a large bowl or the bowl of a stand mixer, cream the butter and sugar together. Add the eggs, one at a time, beating after each addition.

With the mixer on low, add one third of the flour mixture, followed by half of the buttermilk mixture. Continue with another one third of the flour, the remaining buttermilk, then the remaining flour.

Divide the mixture evenly between the 6 muffin cups. Bake in the center of the oven until a toothpick inserted in the center comes out clean, 20-25 minutes. Cool for 10 minutes, then remove to a cooling rack to cool completely.

Set the cooling rack over wax paper or parchment paper. In a small bowl, whisk together the powdered sugar and lemon juice. Pour the glaze over the cakes, spreading it with a small knife or offset spatula if needed. Let the cakes sit for 30 minutes before serving.