Start by making your base muffin batter. In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt
In a separate bowl or large measuring cup, whisk eggs, melted butter, milk, sour cream, and vanilla until smooth
Pour wet ingredients into the dry ingredients and stir gently just until combined — don’t overmix! The batter will be thick and slightly lumpy
Divide the batter evenly into 4 bowls (about 1 cup of batter per bowl). Each bowl will make three muffins
For the blueberry muffins, toss flour and blueberries together, then add to the batter and fold the blueberries gently in. See note below if using frozen!
For the lemon poppy seed muffins, stir zest, juice, and poppy seeds into one portion of batter.
For the coffee cake muffins, stir cinnamon into one portion of the batter
For the double chocolate chip muffins, mix cocoa powder and hot water into a smooth paste. Fold the paste and chocolate chips into one portion of batter.
Cover all the batters and place them in the fridge for at least one hour. This is key to getting those domed muffin tops!
While the batter is in the fridge, make your streusel by combining all the ingredients - flour, sugar, melted butter, salt, and cinnamon - in a bowl until crumbly. Set in the fridge until ready to use
Preheat oven to 425°F and line every other cup of a 12-cup muffin tin with muffin liners. We’ll bake 6 muffins at a time, for maximum heat distribution, to help with the final puff
Remove your blueberry muffin and coffee cake batter from the fridge.
Spoon half of your coffee cake batter evenly across 3 lined muffin cups, then sprinkle over 1 tsp streusel in the middle, and top with the remaining batter (fill to the top)
Divide your blueberry muffin batter between the remaining 3 muffin cups. Split the remaining streusel across the top of the blueberry and coffee cake muffins, pressing in lightly
Bake for 5 minutes, then without opening the oven, drop the temperature to 350°F and bake for an additional 13-17 minutes until a toothpick inserted only has a few moist crumbs. Let the muffins cool in the pan for 5 minutes before transferring to a wire rack to cool completely
Return your oven to 425°F and re-line your muffin pan, every other cup for a total of 6 muffin liners
Divide your lemon poppy seed muffins across three muffin liners, then your double chocolate chip muffins across the remaining three. Press a few extra chocolate chips on the top of the double chocolate muffins
Place in the oven and bake for 5 minutes, drop the temperature to 350°F and bake for an additional 13-17 minutes. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely
Enjoy!