Step 1: Preheat oven to 350°F (175°C). Grease and flour a 9-inch springform pan or a 9x13 inch baking dish. Set aside.
Step 2: In a medium bowl, toss the diced Honeycrisp apples with 1/2 teaspoon of cinnamon and the nutmeg. Set aside.
Step 3: In a separate medium bowl, whisk together 1 3/4 cups flour, baking powder, baking soda, and salt. Set aside.
Step 4: In a large bowl, cream together the softened butter and 1 cup granulated sugar with an electric mixer until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined. Do not overmix.
Step 6: Gently fold in the spiced diced apples into the cake batter.
Step 7: Pour the batter into the prepared baking pan, spreading evenly.
Step 8: To make the streusel topping: In a small bowl, combine 1/2 cup flour, 1/4 cup granulated sugar, brown sugar, and 1/4 teaspoon cinnamon. Cut in the cold butter pieces with your fingertips or a pastry blender until coarse crumbs form.
Step 9: Sprinkle the streusel topping evenly over the cake batter.
Step 10: Bake for 50-60 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. If the streusel browns too quickly, you can loosely tent the cake with foil.
Step 11: Let the cake cool in the pan on a wire rack for at least 15-20 minutes before serving. Enjoy warm or at room temperature.