Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease the cups.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.
In a separate bowl, mix the melted butter, egg, vanilla extract, and buttermilk until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix; a few lumps are okay.
Fold in the chopped strawberries and rhubarb until evenly distributed throughout the batter.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
In a small bowl, combine the brown sugar and cinnamon. Sprinkle this mixture evenly over the tops of the muffins.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.