Preheat oven to 350°F (static) or 325° F (convection). Line 16-18 regular muffin tins with muffin papers, or 10-12 larger muffin papers.
Zest one lemon, then juice it and add to the milk. Set aside to allow time for it to curdle.
Beat together sugar and butter until light; if using my gfJules Muffin Mix, add 1/2 of the mix with the butter, as well as 1/4 sugar if using.
Add egg (or egg substitute), yogurt and vanilla extract.
Whisk dry ingredients together (no need for this step if using my gfJules Muffin Mix). Add zest and stir.
Slowly add dry ingredients to wet ingredient bowl, alternating with milk and lemon juice mixture.
Fold in the berries or poppyseeds.
Fill muffin cups 2/3 full and smooth tops with a rubber spatula dipped in water for a smoother texture.
Bake for 20- 25 minutes for regular muffins; 30-35 minutes for larger muffins as pictured. The tops should be lightly browned and springy to the touch and a toothpick inserted into the centers should come out dry. Remove to cool on a wire rack.
Once cooled, store in a zip-top bag or in a Tupperware container. They will remain soft and fresh for several days; do not store in the refrigerator.