Preheat your oven to 375 degrees F and grease 1-2 muffin tins or line with cupcake liners.
In a mixing bowl, whisk together the flour, baking powder, and salt.
Set aside.
In another mixing bowl, cream together the butter and sugar until fluffy.
Beat in the egg, vanilla, lemon extract, yogurt, lemon zest, and lemon juice.
Add the dry ingredients a little at a time until combined, scraping down the sides of the bowl as needed.
Turn off your mixer and fold in the blackberries until just combined.
In a separate, small bowl, use a fork to mix together the melted butter, sugar, flour, and salt for the crumb mixture.
Fill the muffin tins around ⅔ full with the batter.
Spoon a tablespoon or two of the crumb mixture over the batter onto each muffin.
Place your muffin tins into the oven and bake for 22-24 minutes.