01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease thoroughly with butter or cooking spray.
02 - In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.
03 - In a separate medium bowl, whisk the buttermilk, vegetable oil, eggs, and vanilla extract until smooth and fully incorporated.
04 - Pour the wet mixture into the dry ingredients. Using a rubber spatula, fold gently until just combined. The batter should remain slightly lumpy. Avoid overmixing to prevent tough muffins.
05 - Gently fold in the diced strawberries and rhubarb until evenly distributed throughout the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each approximately 3/4 full.
07 - In a small bowl, combine the flour, brown sugar, cinnamon, and salt. Add the cold cubed butter and rub together with your fingertips or pastry cutter until the mixture resembles coarse crumbs.
08 - Sprinkle the streusel mixture evenly over the batter in each muffin cup, pressing lightly to adhere.
09 - Bake for 22-25 minutes until golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
10 - Allow muffins to cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely.