For best results, use cold butter for the streusel. This will create a crumbly texture. If you don't have a pastry blender, you can use two knives or your fingertips to cut the butter into the flour mixture. Be careful not to overmix the cake batter, as this can result in a tough cake. Feel free to use other varieties of apples, such as Honeycrisp or Fuji, for a slightly different flavor. Store leftover cake in an airtight container at room temperature for up to 3 days.