you can accomplish this by copying the codes from link.phtml to cart.phtml but with a slight modification.
<?php if ($this->isPossibleOnepageCheckout()):?>
<button type="button" title="<?php echo $this->__('Commander') ?>"
class="button btn-proceed-checkout btn-checkout
<?php if ($this->isDisabled()):?> no-checkout
<?php endif; ?>"<?php if ($this->isDisabled()):?> disabled="disabled"
<?php endif; ?> onclick="window.location='<?php echo $this->getCheckoutUrl() ?>';">
<span><span><?php echo $this->__('Commander') ?></span></span></button>
<?php endif?>
Change this to the following:
<?php $block = $this->getLayout()->createBlock('checkout/onepage_link');
if ($block->isPossibleOnepageCheckout()):?>
<button type="button" title="<?php echo $block->__('Proceed to Checkout') ?>" class="button btn-proceed-checkout btn-checkout<?php if ($block->isDisabled()):?> no-checkout<?php endif; ?>"<?php if ($block->isDisabled()):?> disabled="disabled"<?php endif; ?> onclick="window.location='<?php echo $block->getCheckoutUrl() ?>';"><span><span><?php echo $block->__('Proceed to Checkout') ?></span></span></button>
<?php endif?>
Hope it helps.