Prepare a 13×18-inch baking sheet pan (33×45 cm or 38×38 cm) / half-sheet pan. Grease it thoroughly with soft butter.
1 tablespoon soft butter
Preheat the oven to 425°F (220°C). Arrange the wire rack one shelf above the center.
Add all the wet ingredients into a large bowl and whisk until combined.
4 large eggs, zest grated from 2 lemons, 1 tablespoon lemon juice, 2 teaspoons vanilla extract, 1 cup (240g) buttermilk, 1/4 cup + 2 tablespoons (80g) sugar, 1 tablespoon melted butter
Place a sieve over the bowl, sift all the dry ingredients into the wet ingredients. Whisk until just combined, don't overmix the batter.
2 1/4 cups (280g) flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/8 teaspoon salt
Pour the batter into the prepared baking sheet and even out with a spatula. Make sure there is just slightly less batter in the middle than on the edges (edges cook quicker than the middle).
Scatter the blueberries on top of the batter, then place thin slices of butter on top (the remaining three tablespoons).
2 cups (250g) blueberries, 3 tablespoons cold butter
Bake the pancake for about 13-15 minutes. Please keep an eye on the pancake so it won't brown too much. Baking time may vary, depending on the oven and baking sheet dimensions. The pancake is ready when it's set in the middle and golden in color. You can test the center of the pancake with a cake tester or a wooden skewer.
Cut the pancake into squares and serve.
Enjoy!