Preheat oven to 425 degrees F.
Spray three 6-cavity doughnut pans (OR two 4-cavity jumbo doughnut pans) with non-stick cooking spray or wipe with vegetable oil.
Add melted butter, vegetable oil, and granulated sugar to large mixing bowl. Whisk until well combined.
Add eggs, vanilla bean paste, fresh lemon zest and buttermilk to sugar mixture. Whisk until well combined.
Add all-purpose flour, baking powder, baking soda, and fine sea salt to small bowl. Whisk to combine.
Add flour mixture to large mixing bowl. Fold in with large spatula.
Add diced fresh strawberries to doughnut batter. Fold in with large spatula.
Spoon or pipe strawberry doughnut batter into doughnut pans.
Bake regular size doughnuts for 7 to 8 minutes. Bake jumbo size doughnuts 10 to 12 minutes. Doughnuts should be firm to the touch.
Cool in pans for several minutes. Remove doughnuts from pans and cool completely on wire rack.
Brush several with melted butter and sprinkle with granulated sugar.