Let's start by calculating the total cost of the Granny Smith apples. Since Dahlia bought 6 Granny Smith apples at $1.35 each, we can calculate the total cost for those apples:
\[ \text{Total cost of Granny Smith apples} = 6 \times 1.35 = 8.10 \text{ dollars} \]
Now, we know that the total cost of both types of apples (Granny Smith and Macintosh) is $15. Therefore, we can find out how much she spent on the Macintosh apples by subtracting the cost of the Granny Smith apples from the total cost:
\[ \text{Total cost of Macintosh apples} = 15 - 8.10 = 6.90 \text{ dollars} \]
Now we know that Dahlia also bought 6 Macintosh apples. To find the cost of each Macintosh apple, we divide the total cost of the Macintosh apples by the number of Macintosh apples:
\[ \text{Cost of each Macintosh apple} = \frac{6.90}{6} = 1.15 \text{ dollars} \]
Thus, each Macintosh apple costs \( \boxed{1.15} \).