Preheat the oven to 350 degrees F. Line a 9×13 inch pan with parchment paper and set it aside.
Chop the apples and set aside.
First, in a large bowl, combine all the streusel ingredients, and stir with a fork until well blended, and set aside.
Next, using an electric mixer, whisk the sugar and eggs for 1-2 minutes until slightly frothy. Add the melted butter and milk and whisk on low speed until combined.
After that, add the flour, baking powder, cinnamon, nutmeg, and vanilla, and mix on the lowest speed until just combined, do not overmix.
Pour the batter into the prepared pan.
Spread the diced apples over the cake, then crumble the pecan streusel mixture on top.
Bake until lightly browned for about 45 to 50 minutes.
Cool on a wire rack before slicing and serving.