Preheat oven to 425°F and lightly grease or line an 8x8 baking pan.
In a medium bowl, mix streusel ingredients and stir in melted butter until crumbly; set aside.
In a large bowl, whisk eggs, oil, brown sugar, white sugar, buttermilk, and vanilla until smooth.
Add gluten free flour, baking soda, baking powder, cinnamon, and salt; mix gently until just combined.
Spread the batter evenly in the pan and top with streusel; press gently.
Bake for 7 minutes at 425°F, then reduce temperature to 350°F and bake for 30–35 minutes more.
Let cool for 15–20 minutes before slicing.
This gluten free coffee cake recipe is best served warm and pairs beautifully with coffee or tea.