Preheat your oven to 350°F (175°C). Line your muffin tin with cupcake liners to prepare for the batter.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until well combined. Set aside.
In another bowl, beat the softened butter and granulated sugar together until the mixture is light and fluffy. This should take about 3-4 minutes using an electric mixer.
Add the large eggs, one at a time, beating well after each addition. Then, mix in the vanilla extract until fully incorporated.
Blend in the vegetable oil and milk into your butter-sugar mixture, stirring until smooth.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Be careful not to overmix!
Gently fold in the instant pistachio pudding mix and crushed pineapple, ensuring everything is evenly distributed.
Scoop the batter into the lined muffin tin, filling each liner about two-thirds full. Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Once baked, remove the cupcakes from the oven and allow them to cool in the tin for about 5 minutes. Then, transfer them to a wire rack to cool completely.
In a mixing bowl, beat together the softened butter, marshmallow creme, confectioners' sugar, vanilla extract, and milk until the frosting is smooth and fluffy.
Once the cupcakes are completely cooled, generously frost each with the marshmallow frosting, and garnish with chopped pecans for that extra crunch.