Make the Raspberry Filling: In a small saucepan, combine raspberries, sugar, and lemon juice over medium heat. Stir until raspberries break down. Whisk cornstarch with 1 tbsp of water to make a slurry, then add to the saucepan. Cook for 1-2 minutes until thickened. Remove from heat and cool completely.

Prepare for Baking: Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with liners.

Combine Dry Ingredients: In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

Combine Wet Ingredients: In a large bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, followed by vanilla and oil. Mix well.

Make the Batter: Alternate adding the dry ingredients and buttermilk to the wet ingredients, starting and ending with the dry. Mix until just combined.

Bake: Fill cupcake liners two-thirds full. Bake for 18-22 minutes, or until a skewer comes out clean. Let cool completely.

Make the Frosting: Melt the white chocolate and let it cool slightly. Beat softened butter until creamy. Gradually add powdered sugar and beat until smooth. Mix in the cooled white chocolate, vanilla, and salt. Add heavy cream, 1 tablespoon at a time, until you reach a light, pipeable consistency.

Assemble the Cupcakes: Once cupcakes are cool, core the center of each. Fill the hole with the cooled raspberry filling. Pipe the white chocolate buttercream on top. Garnish with a fresh raspberry if desired.