Preheat to 275°F (fan) or 320°F (conventional). Line two cupcake trays with paper liners.
In a large bowl, whisk together flour, caster sugar, baking powder, and salt. Add softened butter and mix until it looks like fine crumbs.
In another bowl, whisk eggs, milk, oil, yogurt or sour cream, vanilla, and tequila (if using) until smooth.
Pour the wet mix into the dry mix. Stir until smooth. Divide the batter into two bowls.
Add pink/red food gel and strawberry flavor to one bowl. Add yellow food gel and mango flavor to the other. Mix gently.
Layer both batters into each cupcake liner using spoons or piping bags. Fill them about ⅔ full.
Step 7: Bake
Bake for 40–50 minutes. A toothpick should come out clean. Cool for 10 minutes, then move to a rack.
Prepare your buttercream. Split into two bowls. Tint one pink/red with strawberry flavor, the other yellow with mango flavor.
Brush cooled cupcake tops with water. Roll them in granulated sugar for a “salted rim” effect.
Pipe both frostings together in a swirl. Top each cupcake with a lime wedge and half a strawberry.