## Step 1: Define the symbolic representation of the problem

Let's denote:
- $x_1$ as the amount of vanilla cake mix in kg,
- $x_2$ as the amount of chocolate cake mix in kg.

The objective is to minimize the cost, which is $10x_1 + 15x_2$.

## Step 2: Translate the constraints into symbolic notation

The constraints given are:
- The final mixture needs at least 0.3 kg of leavening agent. Given that vanilla cake mix contains 3% leavening agent and chocolate cake mix contains 2% leavening agent, this translates to $0.03x_1 + 0.02x_2 \geq 0.3$.
- The final mixture needs at least 10 kg of flour. Given that vanilla cake mix contains 55% flour and chocolate cake mix contains 43% flour, this translates to $0.55x_1 + 0.43x_2 \geq 10$.
- Non-negativity constraints: $x_1 \geq 0$ and $x_2 \geq 0$.

## 3: Symbolic representation in JSON format

```json
{
'sym_variables': [('x1', 'vanilla cake mix'), ('x2', 'chocolate cake mix')],
'objective_function': '10*x1 + 15*x2',
'constraints': [
    '0.03*x1 + 0.02*x2 >= 0.3',
    '0.55*x1 + 0.43*x2 >= 10',
    'x1 >= 0',
    'x2 >= 0'
]
}
```

## 4: Convert the problem into Gurobi code

```python
import gurobi

def solve_wedding_cake_problem():
    # Create a new model
    model = gurobi.Model()

    # Define variables
    x1 = model.addVar(name="vanilla_cake_mix", lb=0, ub=gurobi.GRB.INFINITY)
    x2 = model.addVar(name="chocolate_cake_mix", lb=0, ub=gurobi.GRB.INFINITY)

    # Objective function: minimize 10*x1 + 15*x2
    model.setObjective(10*x1 + 15*x2, gurobi.GRB.MINIMIZE)

    # Constraints
    model.addConstr(0.03*x1 + 0.02*x2 >= 0.3, name="leavening_agent")
    model.addConstr(0.55*x1 + 0.43*x2 >= 10, name="flour")

    # Solve the model
    model.optimize()

    # Print the solution
    if model.status == gurobi.GRB.OPTIMAL:
        print("Optimal solution found.")
        print(f"Vanilla cake mix: {x1.varValue} kg")
        print(f"Chocolate cake mix: {x2.varValue} kg")
        print(f"Optimal cost: {model.objVal}")
    else:
        print("No optimal solution found.")

solve_wedding_cake_problem()
```