Preheat oven to 375 degrees. Butter 3 6-hole doughnut tins with softened butter and dust lightly with flour (shaking out excess), set aside.
In a mixing bowl, whisk together flour, baking powder, baking soda, salt and poppy seeds for 30 seconds. In a separate large mixing bowl, using an electric hand mixer set on low speed, blend together melted butter, canola oil, sugar and lemon zest for 1 minute.
Mix in eggs one at a time. In liquid measuring cup used to measure buttermilk, whisk lemon juice with buttermilk (it will begin to curdle a little so you want to mix it in rather quickly).
Working in 3 separate batches, beginning and ending with flour mixture, add 1/3 of the flour mixture alternating with 1/2 of the buttermilk mixture and mixing just until combined after each addition.
Pour mixture into piping bag and pipe mixture (or spoon) into prepared doughnut wells, filling within about 1/3-inch of the top rim.
Bake in preheated oven until toothpick comes out clean, about 7 - 8 minutes. Allow to cool slightly, then invert onto a wire rack to cool.
In a bowl, whisk together powdered sugar and 3 1/2 Tbsp fresh lemon juice to make glaze. Dip tops of doughnuts in glaze and return to wire rack, sprinkle tops with poppy seeds and allow glaze to set at room temperature. Store in an airtight container.