Here is a technique that I have used before and I just love the outcome. It is easy to do and you can get so many different looks.
I made plain black envelope to go with it and just embossed Happy Birthday on the front.
Here is a close up of the main feature of the card. To achieve this I first cut a piece of black building paper to fit my Grand Calibre then covered it with some double sided tape, you can use any brand I have a wide roll of Sookwang Double-Sided Tape. I then scrunched up some aluminium foil and placed it on top and then I put it in between an embossing folder and ran it through my Grand Calibre machine.
Then using a blending tool with Alcohol inks I covered the embossed foil. Then holding the piece vertical I added alcohol ink to the top and let it drip down through the embossing. Lastly I used the Tim Holtz sanding block I rubbed it gently and took off a layer of ink off the raised areas.
I covered chipboard numbers 2 & 1 in silver embossing powder and heat set to achieve a metallic look, then used the same embossing powder to the stamped Happy Birthday on the black cardstock. Attached the 21 to the cardstock, added two brads in the corners and backed it with another piece of cardstock. I then layered it all together.
On the inside I used some leftovers to make a gift card holder & some detail on the other side.