01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, whisk buttermilk, vegetable oil, eggs, and vanilla extract until smooth and combined.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined, being careful not to overmix.
05 - Gently fold diced strawberries and rhubarb into the batter until evenly distributed.
06 - Divide batter evenly among muffin cups, filling each about three-quarters full.
07 - In a small bowl, mix flour, brown sugar, cinnamon, and salt. Add cold butter and rub together with fingers or pastry cutter until mixture resembles coarse crumbs.
08 - Sprinkle streusel mixture evenly over the batter in each muffin cup.
09 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out clean.
10 - Let cool in pan for 5 minutes, then transfer to a wire rack to cool completely.