Preheat oven to 375 and grease your muffin tin with non-stick spray.
Make the streusel topping if using. Combine all the ingredients with your fingertips until is crumbly and the butter is completely incorporated. Set aside.
In a large mixing bowl whisk together the eggs, oil, sugars, and vanilla extract. Add Bourbon if using.
Add baking soda, baking powder, and salt to the all-purpose flour.
Gradually add the flour to the mixture, alternating with the buttermilk. Begin and end with the flour. Fold until just combined.
Be careful not to overmix; the batter should be slightly lumpy to ensure tender muffins.
Gently fold the Marionberries into the batter. This step ensures the berries are evenly distributed throughout the muffins.
Spoon the batter into the prepared muffin cups, filling each about 3/4 full. Add streusel on top of each muffin before baking.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Allow the muffins to cool in the muffin pan for about 5 minutes, then transfer them to a wire rack to cool completely.
Notes
Nutritional information is provided as a courtesy and is an estimate only. The information comes from online calculators.