Prepare a 10x17-inch or 10x15-inch / 34x34 cm or 25 x 38 cm sheet pan or similar, grease it thoroughly with a tablespoon of butter.
Preheat the oven to 425°F / 220°C / Gas Mark 7, no fan (if you have a convection/fan oven reduce the temperature by about 20° or follow the manufacturer's instructions).
Add all the wet ingredients (egg, zest and lemon juice, vanilla extract, buttermilk, sugar, and oiinto a large bowl, whisk until combined.
Place a sieve over the bowl, sift the dry ingredients (flour, baking powder, baking soda, and salt) into the wet ingredients. Whisk until just combined, don't overmix the batter.
Pour the batter into the prepared baking pan and smooth with a spatula.
Scatter the blueberries on top of the batter, then place thin slices of butter on top (the remaining three tablespoons).
Bake the pancake for about 10 minutes. I like to place the baking sheet one shelf above the middle shelf or on the top shelf, so it's golden on top, not pale. 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 no more jiggly in the middle and lightly golden on the sides. You can test the center of the pancake with a cake tester or a wooden skewer.
Cut the pancake into squares, serve with maple syrup, a dollop of yogurt/whipping cream and fresh fruits.