Preheat oven to 350°F (175°C). Grease a 9-inch square pan.
Mix dry ingredients, then beat butter and eggs. Alternate adding dry mix and milk. Fold in raspberries.
Pour batter into pan and bake for 30-35 minutes. Cool completely.
Make lemon buttercream by beating butter, then gradually adding powdered sugar, lemon juice, and zest. Adjust with heavy cream for desired consistency.
Frost cooled cake with lemon buttercream and top with extra raspberries.
Notes
For best flavor, use fresh raspberries and freshly squeezed lemon juice.