Preheat the oven to 175°C (350°F) and line the muffin tin with paper cupcake liners.
In a medium bowl whisk together the flour, baking powder, and salt until well combined.
In a large bowl whisk together the sugar, milk, oil, eggs, and vanilla until smooth.
Add the dry ingredients to the wet ingredients and whisk just until no dry streaks remain.
Stir in the hot water until the batter is smooth and slightly thin; do not overmix.
If using lavender, fold the crushed dried lavender into the batter at this stage.
Divide the batter evenly among the 12 liners, filling each about two-thirds full.
Bake the cupcakes for 16–18 minutes or until a toothpick inserted in the center comes out clean.
Transfer the pan to a wire rack and let the cupcakes cool in the tin for 5 minutes.
Remove the cupcakes from the tin and let them cool completely on the wire rack, about 40 minutes.
While the cupcakes cool, beat the softened butter with an electric mixer on medium speed until pale and fluffy, about 3 minutes.
Gradually add the sifted powdered sugar, mixing on low speed until incorporated, then increase to medium until smooth.
Beat in the vanilla, salt, and heavy cream until the frosting is light and spreadable.
If using rose water, beat it into the frosting now, adding a few drops at a time and tasting as you go.
Divide the frosting evenly into three bowls for different flower colors.
Tint one bowl of frosting pink, one yellow, and one green by adding a few drops of gel food coloring and mixing well.
Fit three piping bags with a petal tip, a round or star tip, and a leaf tip respectively.
Fill the petal-tip bag with pink frosting, the round or star-tip bag with yellow frosting, and the leaf-tip bag with green frosting.
Once the cupcakes are completely cool, spread a thin base layer of plain or lightly tinted frosting on each cupcake.
Pipe flower petals with the petal tip, holding the wide end of the tip toward the cupcake surface and rotating slightly as you pipe.
Add centers to the flowers using the yellow frosting with the round or star tip in small dots or swirls.
Pipe leaves and small stems with the green frosting using the leaf tip to fill gaps and frame the flowers.
Repeat the decorating process on all cupcakes, varying colors and flower styles for a mixed bouquet look.
Let the decorated cupcakes rest at cool room temperature for at least 15 minutes so the frosting can set slightly before serving.