Preheat oven to 375°F and prepare a 6" cake pan by greasing and lining with parchment.
In a small bowl, whisk together all-purpose flour, baking powder, salt, and ground cinnamon and set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together unsalted butter and granulated sugar on medium-high speed until pale in color and fluffy, approximately 3-5 minutes.
To the butter and sugar, add the egg, vanilla extract, and lemon juice, and mix to combine on medium speed. Then, add flour mixture and mix on low speed until just combined. Fold in diced apple with a stiff rubber spatula or wooden spoon. Pour batter into prepared cake pan, and use a spoon or offset spatula to spread it evenly.
Bake until golden brown around the edges and a toothpick inserted in the middle comes out clean, approximately 25-30 minutes.
Remove from the oven, and let cool in the pan for 15 minutes before transferring to a wire rack to continue cooling.