Preheat the oven to 425°F. Add 18 paper liners to two muffin tins.: The warm oven fills the kitchen with a hint of heat and sets you up for an immediate rise once the batter hits the pan, giving a quick burst of steam that encourages lofty muffins. You should hear the quiet hum of the oven settling as it reaches 425°F , and you may notice a faint toasted scent when it's ready. This high initial heat creates an oven spring, which is important for a domed top and tender interior. A common mistake is not preheating long enough, which leads to flat muffins, or placing the tins on a cold rack which causes uneven bake. Make sure both pans are centered so heat circulates evenly.
In a large bowl, whisk together the flour, baking powder, salt, and sugar.: The dry mix should look uniform, with tiny specks of leavening dispersed through the all purpose flour . The whisking action aerates the mixture, which contributes to lighter texture. You may notice a faint flour scent that will become more aromatic as wet ingredients are added. Doing this step thoroughly prevents pockets of baking powder or salt, which can cause bitter or uneven spots. One slip is using a packed cup of flour, which will yield dense muffins, so fluff and level when measuring.
In a small bowl or measuring cup, whisk together the buttermilk, oil, melted butter, eggs, and vanilla. Pour the wet ingredients into the dry ingredients, and mix until just barely combined.: When you whisk the buttermilk , oil , melted butter , and eggs , the mixture will glisten and smell gently of butter and vanilla, indicating the fats are well integrated. Pouring the wet into the dry should create a shaggy batter with small streaks of flour, not a uniformly smooth paste. This under mixing preserves a tender crumb, because overworking develops gluten and makes muffins tough. Watch for one common pitfall, vigorous stirring, which tightens the batter; fold with a soft spatula to avoid that.
Gently fold the strawberries into the muffin batter. Scoop the batter into the prepared muffin tins, filling 3/4 full.: As you fold, you will feel soft resistance from the batter and see bright flecks of strawberries appear, giving a promise of juicy bites. Folding distributes fruit without breaking it down excessively, preserving texture and preventing pink streaks from taking over the batter. Use a scoop or large spoon to fill each liner evenly, which helps the muffins bake at the same rate. A frequent mistake is overfilling the cups, which causes spillover, or underfilling, which yields small muffins. Aim for that three quarters level for best rise.
To make the streusel, whisk the flour, sugar, and brown sugar together with a fork in a small bowl. Drizzle in the melted butter and whisk until the butter is blended in and the mixture is crumbly.: The streusel should transform into coarse crumbs that glisten slightly from the melted butter . Rubbing the mixture together with a fork helps achieve those rustic clusters that will crisp during baking. The aroma will shift to a buttery caramel scent thanks to the brown sugar . If the mixture becomes pasty rather than crumbly, you have added too much butter, which will make the topping greasy instead of crispy. Keep the texture loose and crumbly for the best finish.
Sprinkle the streusel over the tops of the muffins and bake 15 minutes, or until light golden brown on top and cooked through.: As the muffins bake, you will see the streusel take on color and hear a faint crackling as sugars caramelize. The tops should turn a light golden brown, and the kitchen will fill with a warm, sweet aroma that signals readiness. To check doneness, press gently on a top it should spring back, or insert a toothpick into the center and expect a few moist crumbs but no wet batter. A common error is baking too long at this temperature which dries them out, so start checking at the lower end of timing and adjust for your oven's behavior.