Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch sheet cake pan.
In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 3 to 4 minutes.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Alternately add the flour mixture and the milk to the butter mixture, beginning and ending with the flour mixture. Mix until just combined.
Fold in the sour cream gently until the batter is smooth.
Pour the batter evenly into the prepared sheet cake pan and smooth the top with a spatula.
Bake for 32 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool completely in the pan on a wire rack.
To make the buttercream, beat the softened butter with an electric mixer on medium speed until creamy, about 2 minutes.
Gradually add the powdered sugar, one cup at a time, beating well after each addition.
Add the heavy cream and vanilla extract, and beat on high speed for 3 to 5 minutes until the buttercream is light and fluffy.
Divide the buttercream into two portions: place about 1/3 in a small bowl and tint it green with gel food coloring for leaves.
Tint the remaining larger portion red with gel food coloring for roses.
Fill a piping bag fitted with a rose petal tip with the red buttercream and another piping bag fitted with a leaf tip with the green buttercream.
Pipe buttercream roses evenly across the top of the cooled cake by starting with a small cone shape and swirling petals around it.
Pipe green leaves around the roses to complete the decoration.
Refrigerate the decorated cake for at least 30 minutes before serving to set the buttercream.