IDA Pro

Debug a DLL:

1. Make an Ida project from the DLL, i.e. drag and drop the dll into the blank Ida page.

2. In Menu Debugger, Process Options, put the path to your exe into the textbox "Application", Into "input file" put the path to your DLL. Confirm with OK.

3. Start with menu Debugger, Start Process or F9.

Source: https://reverseengineering.stackexchange.com/questions/9328/how-to-debug-the-dll-of-an-exe-using-ida-pro