For the curd, combine raspberries, C&H® Granulated Sugar, and salt in a medium saucepan. Bring to a simmer over moderate heat, stirring until sugar is dissolved and raspberries are broken down.
Strain through a fine-mesh sieve into a bowl, pressing firmly on seeds. Return raspberry sauce to saucepan. Combine water, lemon juice, and arrowroot starch. Whisk together until completely smooth, then add to saucepan and stir in.
Bring to a simmer over medium-low heat, stirring until mixture begins to thicken. Add butter a few pieces at a time, whisking constantly until melted, before adding more butter.
When curd is smooth and thickened, and coats the back of a spoon, transfer to a glass bowl. Press a piece of plastic directly on the surface, and chill until cold – at least one hour.
Preheat oven to 350°F and line two 12-cup standard muffin pans with baking cups.
Whisk together cake flour, baking powder, baking soda, and salt in a medium bowl. Beat butter and C&H® Granulated Sugar in a large mixer bowl at medium speed until light and fluffy; beat in lemon zest and vanilla. Add eggs one at a time and beat until combined. Combine buttermilk and yogurt in a small bowl. Reduce speed to low and add flour mixture in 3 additions, alternating with buttermilk mixture (begin and end with flour mixture).
Use a medium cookie scoop coated with non-stick spray to fill each baking cup 2/3 full of batter.
Bake for 18 to 20 minutes, until cupcakes spring back to the touch and a toothpick inserted in the center comes out clean. Let cupcakes cool in pans on wire racks for 2 minutes before removing from pans to cool completely.
Beat butter and salt in a large mixer bowl on medium speed until smooth. Beat in vanilla, lemon zest, and lemon juice. Beat in raspberry curd until combined. Gradually mix in C&H® Confectioners Sugar on low speed until incorporated, scraping down bowl as needed. Add heavy cream and mix in on low speed until smooth. Mix in raspberries. Increase speed to medium-high and beat frosting for 1 minute, until light and airy.
To fill the cupcakes, use a small paring knife to cut a small cone out of the center of each cupcake. Fill each cupcake with a spoonful of raspberry curd. Transfer frosting to a large piping bag fitted with a large star tip and decorate cupcakes. Chill 20 minutes before serving.
Store leftover cupcakes covered in the refrigerator up to 3 days. Bring to room temperature before serving. If desired, decorate cupcakes with fresh raspberries and lemon slices.