Preheat oven to 350°F (175°C). Grease or line a 9x13-inch baking dish with parchment paper, leaving overhang for easy removal.
Cream butter and sugar for 2–3 minutes until light and fluffy. Mix in eggs and vanilla.
In another bowl, whisk flour, cream of tartar, baking soda, salt, and cinnamon. Gradually mix dry ingredients into wet until a soft dough forms.
Press half the dough evenly into the baking dish using a spatula or the back of a spoon.
Toss diced apples with sugar, flour, cinnamon, nutmeg, and lemon juice until evenly coated.
Spread the apple mixture evenly over the dough, covering the entire surface.
Drop spoonfuls of the remaining dough over the apples. Lightly spread with a spatula or fingers to cover most of the filling.
Mix sugar and cinnamon in a small bowl. Sprinkle the mixture evenly over the top.
Bake for 30–35 minutes until golden on top and a toothpick (not touching apples) comes out clean.
Cool completely in the pan. Once set, lift out using parchment and slice into squares.