Preheat oven to 350 degrees F. Line muffin tins with paper liners.

Sift cake flour, baking powder, and salt into the bowl of a stand mixer. Add the lemon zest, and, using the paddle attachment, mix together briefly on low speed.

With the mixer on medium-low, add the butter, one piece at a time, letting each piece incorporate before adding the next. Mix in the sugar and then increase the speed to medium and mix until the mixture looks like fine, sandy crumbs with a few larger pieces, about 3 minutes.

Add in the eggs, one at a time, beating well after each addition.

Combine the milk, lemon juice, and vanilla in a liquid measuring cup. Pour the milk mixture into the batter and beat on medium speed for about 1 minute, scraping down the sides of the bowl as needed.

Scoop batter into cupcake wells, filling each a little over half full. Bake cupcakes for 14-15 minutes, until a tester inserted into the center comes out with just a few crumbs attached. Let cool in pan for 10 minutes, then remove to wire rack to cool completely before filling and frosting.

While the cupcakes bake, puree thawed raspberries with 2 tbsp. water in a blender until smooth. Strain through a fine-mesh strainer into a small saucepan to remove the seeds. Stir in the sugar and cornstarch and bring to a simmer over low heat. Cook until thickened, stirring constantly with a small spatula, about 1 minute. Remove from heat and let cool.

Use a small paring knife to scoop a small wedge out of the top center of each cupcake. Put the raspberry puree into a piping bag fitted with a small tip. (Alternatively you can use a small spoon to scoop the filling into the cupcakes.) Pipe a small amount of filling into the top of each cupcake.

Melt the chopped chocolate in the microwave by heating it for one minute, stirring, and then heating in 15-second intervals until melted, stirring until smooth. Set aside to let cool.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until pale and creamy, about 4 minutes. Add the powdered sugar, cocoa powder, salt, and vanilla and beat on medium speed for 3 minutes, scraping down the sides of the bowl as needed.

Mix in the Greek yogurt and melted chocolate and beat on medium speed until light and fluffy, about 4 minutes. Frost cupcakes as desired and top with fresh raspberries.