Heat oven to 400 Degrees F. Line two jumbo muffin tins with 9 paper muffin liners - tulip cups work great here - you can make your own with 6x6 inch square pieces of parchment paper. Place the square over the each cup then place a glass such as a small mason jar or a pint glass and press into the molds. Or you can fold them over the bottom of an upside down glass that is the same diameter as the muffin tin. They might pop back out a little but they will settle once filled with batter. ( see my Meyer lemon crumb muffins on my blog for photos of diy tulip cups)
Make the crumb topping:
In a bowl combine the sugar, flour, melted butter and lemon zest and mix together with a fork until it all comes together, crumble into pea sized pieces and chill until set. I pop this in the freezer to set up fast. You will crumble again right before adding to the muffins.
100 g granulated white sugar, 100 g flour, 113 g butter, 1 tablespoon lemon zest
Make the muffins:
In a medium bowl whisk together the flour, baking powder and salt. Mix the blueberries with the 30 g of ap flour (two tablespoons). You can rinse the blueberries and dry slightly then add the flour so it sticks better.
390 g AP flour, 1 Tablespoon baking powder, ½ teaspoon kosher salt, 250 g fresh blueberries, 30 g Ap flour
In a large bowl using a hand mixer or a stand mixer with the paddle attachment combine the butter, oil, sugars and lemon zest and mix until light and fluffy and smooth, 3-4 minutes.
84 g unsalted butter, 26 g neutral oil, 200 g granulated white sugar, 66 g light brown sugar, 2 tablespoons lemon zest
Add in the egg, yolks and vanilla and mix for another 3 minutes.
1 large egg plus two large yolks, 2 teaspoons vanilla extract
In a measuring cup whisk together milk and sour cream.
180 ml whole milk, 100 g sour cream
Alternate adding in the flour mixture with the milk mixture with your hand mixer or stand mixer on low speed. Once everything is added, scrape down the sides of the bowl to make sure all the flour is incorporated then mix for one minute more on low speed.
Add the blueberries and any remaining bits of flour and stir them in to the batter with a spatula carefully. Do not overmix.
Divide the batter between the 9 cups (About 160 grams of batter each)
If using lemon curd, swirl a tablespoon or so into the top of the batter, then divide the crumb mixture over the tops of the muffins
Optional - lemon curd
Bake for 22-25 minutes or until a toothpick or skewer inserted into the middle comes out with a few crumbs)
Let the muffins cool a bit before eating. They are great still warm or room temperature