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Preheat the oven to 350°F (180°C). Butter and line with parchment an 8-inch (20 cm) round cake pan. Set aside.
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In a medium bowl, combine the oat flour, all-purpose flour, cinnamon, nutmeg, baking powder, baking soda and salt.
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In a separate medium bowl, whisk together the eggs, grated apple, brown sugar, buttermilk, oil and vanilla extract.
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Combine the wet and dry ingredients until just combined, then spread into your prepared pan. Crumble the streusel topping over the batter.
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Bake for 45-55 minutes, or until a skewer inserted into the center comes out clean.
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Let cool for 15 minutes before removing from the pan and slicing. Store leftovers covered at room temperature for up to 3 days.