Preheat the oven to 350°F / 175°C. Line a muffin tin with 6 parchment liners.
To a medium sized mixing bowl, add the flour, baking powder, salt, sugar, and butter. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and no lumps remain.
Add in the oil, egg white, vanilla, sour cream and milk. Whisk the ingredients together just until a cohesive batter has formed.
Distribute the batter evenly between the six liners. Bake for 15-20 min., or until a toothpick inserted in the center of the cupcakes comes out clean.
Cherry Jam
Rinse the cherries and pit them.
Add the cherries, sugar, and lemon juice to a small saucepan set over medium heat. Stir and mash down the cherries ocasionally, until the mixture comes to a simmer.
In a small bowl, whisk together the cornstarch and water.
While constantly whisking the cherries, pour the cornstarch slurry into the saucepan. Continue to constantly whisk until the mixture thickens (~1 minute.)
Remove the cherry jam from the heat and place it in the fridge to cool down before filling the cupcakes.
White Chocolate Buttercream
In a small mixing bowl, melt the white chocolate and set aside.
In a medium sized mixing bowl, beat together the butter and salt for one minute.
Add in half of the powdered sugar and beat until combined, then add in the rest of the powdered sugar. Beat in the melted white chocolate.
Cupcake Assembly
Push down in the center of the cupcakes to make a divot for the cherry jam, or use a large round piping tip to scoop out the center of the cupcakes.
Fill the inside of each cupcake with some of the cherry jam.
Transfer the white chocolate buttercream to a piping bag fitted with your tip of choice, and generously frost each cupcake.
Top each cupcake with extra cherries. Serve and enjoy!