Adapted from The Best of America's Test Kitchen For streusel topping: 1 1â„4 cups flour 65 grams (1â„3 cup) brown sugar 75 grams (1â„3 cup) granulated sugar Pinch of salt 100 grams (7 Tbsp.) butter, melted For muffins: 1 large egg 1 tsp. vanilla 1 cup sugar 4 Tbsp. butter, melted 1â„2 cup buttermilk (Rivyon) 2 cups flour 1 Tbsp. baking powder 1â„2 tsp salt 1 1â„2 cups frozen blueberries Preheat oven to 190ÂºC Line the muffin cups with 10-cm. by 10 cm. squares of baking paper so that the paper overhangs each muffin cup. For the topping: Weigh or measure out the dry ingredients into a bowl. Add the melted butter and mix until you get crumbs and the entire mixture is somewhat moistened. Set aside. For the muffins: Take out three medium bowls. In the first, whisk the egg until well beaten and light. Add the vanilla and sugar. Whisk until well combined. Add the butter and whisk until blended. Add the buttermilk and whisk some more. Set aside. In the second bowl, weigh or measure out your flour. Remove one tablespoon of flour and place in the third bowl. Into the larger amount of flour, add the baking powder and salt. Whisk to combine. Add the flour mixture to the egg mixture mix until just combined. Back to the third bowl: Add the frozen blueberries (straight from the freezer) into the flour and toss. Quickly mix the blueberries into the muffin batter. Scoop the batter into the lined muffin tins, top with prepared streusel and pop in the oven for 25 minutes until tops are browned and springy to touch.